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Abstract:

The invention provides a PLC-type DP-QPSK demodulator that reduces
connection loss between a polarization beam splitter and a 90-degree
hybrid circuit and aims at reducing the manufacturing cost and an optical
transmission system using the same. In an embodiment of the invention, a
PLC-type DP-QPSK demodulator that receives a DP-QPSK signal includes one
PLC chip having a planar lightwave circuit. Input ports and output ports
of signal light are provided at an input end and at an output end of the
PLC chip, respectively. Within the planar lightwave circuit, there are
integrated a polarization beam splitter that splits the DP-QPSK signal
into an X-polarization QPSK signal and a Y-polarization QPSK signal, and
two 90-degree hybrid circuits that mix the X-polarization QPSK signal and
local oscillation light and the Y-polarization QPSK signal and local
oscillation light, respectively, split each QPSK signal into orthogonal
components I, Q and output them.

Claims:

1. A PLC-type demodulator that receives and demodulates a
polarization-multiplexed coherent modulated signal, the demodulator
comprising: one PLC chip in which a planar lightwave circuit is formed; a
first input port provided at an input end of the PLC chip and inputting
the polarization-multiplexed coherent modulated signal into the planar
lightwave circuit; a second input port provided at the input end of the
PLC chip and inputting local oscillation light into the planar lightwave
circuit; at least one polarization beam splitter that splits the
polarization-multiplexed coherent modulated signal input from the first
input port into an X-polarization coherent modulated signal and a
Y-polarization coherent modulated signal; a first 90-degree hybrid
circuit that mixes and outputs the X-polarization coherent modulated
signal and the local oscillation light input from the second input port;
and a second 90-degree hybrid circuit that mixes and outputs the
Y-polarization coherent modulated signal and the local oscillation light
input from the second input port, wherein the at least one polarization
beam splitter, the first 90-degree hybrid circuit, and the second
90-degree hybrid circuit are integrated within the planar lightwave
circuit.

2. The PLC-type demodulator according to claim 1, further comprising a
second polarization beam splitter that splits the mixed light of the
X-polarization local oscillation light and the Y-polarization local
oscillation light into the X-polarization local oscillation light and the
Y-polarization local oscillation light, wherein the polarization beam
splitter and the second polarization beam splitter each have an input
side coupler and an output side coupler; the polarization beam splitter
and the second polarization beam splitter are provided so that the input
side coupler is located on the output end side of the PLC chip opposite
to the input end and the output side coupler is located on the input end
side; and the first 90-degree hybrid circuit, the polarization beam
splitter, the second polarization beam splitter, and the second 90-degree
hybrid circuit are arranged in this order in a direction perpendicular to
the direction going from the input end toward the output end, wherein the
PLC-type demodulator further comprises: a waveguide that connects the
first input port and the input side coupler of the polarization beam
splitter and has a bent region so as to fold propagating light; a
waveguide that connects the second input port and the input side coupler
of the second polarization beam splitter and has a bent region so as to
fold propagating light; a first waveguide that connects the output side
coupler of the polarization beam splitter and the first 90-degree hybrid
circuit, transmits one of the X-polarization coherent modulated signal
and the Y-polarization coherent modulated signal, and has a bent region
so as to fold propagating light; a second waveguide that connects the
output side coupler of the polarization beam splitter and the second
90-degree hybrid circuit, transmits the other of the X-polarization
coherent modulated signal and the Y-polarization coherent modulated
signal, and has a bent region so as to fold propagating light; a third
waveguide that connects the output side coupler of the second
polarization beam splitter and the first 90-degree hybrid circuit,
transmits one of the X-polarization local oscillation light and the
Y-polarization local oscillation light, and has a bent region so as to
fold propagating light; and a fourth waveguide that connects the output
side coupler of the second polarization beam splitter and the second
90-degree hybrid circuit, transmits the other of the X-polarization local
oscillation light and the Y-polarization local oscillation light, and has
a bent region so as to fold propagating light, and wherein the optical
path length of the first waveguide and the optical path length of the
second waveguide are the same.

3. The PLC-type demodulator according to claim 2, wherein the optical
path lengths of the first waveguide, the second waveguide, the third
waveguide, and the fourth waveguide are the same; and the second
waveguide and the third waveguide intersect with each other at an
intersection angle 2.theta., wherein the first waveguide has a first bend
waveguide connected to the output side coupler of the polarization beam
splitter, a first straight waveguide connected to the first bend
waveguide, and a second bend waveguide connected to the first straight
waveguide; the second waveguide has a third bend waveguide connected to
the output side coupler of the polarization beam splitter, a second
straight waveguide connected to the third bend waveguide, and a fourth
bend waveguide connected to the second straight waveguide; the third
waveguide has a fifth bend waveguide connected to the output side coupler
of the second polarization beam splitter, a third straight waveguide
connected to the fifth bend waveguide, and a sixth bend waveguide
connected to the third straight waveguide; and the fourth waveguide has a
seventh bend waveguide connected to the output side coupler of the second
polarization beam splitter, a fourth straight waveguide connected to the
seventh bend waveguide, and an eighth bend waveguide connected to the
fourth straight waveguide, wherein the first, third, fifth and seventh
bend waveguides have the same shape as a fan-shaped arc with a bend
radius r and a central angle θ; and the second, fourth, sixth and
eighth bend waveguides have the same shape as a fan-shaped arc with the
bend radius r and a central angle greater than π-2.theta.
(0<θ<π/2), and wherein a length l of the first, second,
third and fourth straight waveguides satisfies a relationship l=(2r cos
θ-r-p/2)/sin θ when an interval between two waveguides in the
proximity of the output side coupler is assumed to be p; and the second
waveguide and the third waveguide intersect with each other at a boundary
between the second straight waveguide and the fourth bend waveguide and
at a boundary between the third straight waveguide and the fifth bend
waveguide.

4. The PLC-type demodulator according to claim 1, wherein a path through
which the X-polarization coherent modulated signal propagates and a path
through which the Y-polarization coherent modulated signal propagates are
set so that all the effective optical path lengths from the input end
toward the output end of the PLC chip are the same.

5. The PLC-type demodulator according to claim 1, wherein the number of
the polarization beam splitters is two or more, and the polarization beam
splitters and the first and second 90-degree hybrid circuits are
arranged, respectively, in the proximity of each other.

6. The PLC-type demodulator according to claim 5, wherein the
polarization beam splitters are cascade-connected in two or more stages.

7. The PLC-type demodulator according to claim 1, wherein the PLC chip is
rectangular, substantially close to square, in shape; a polarization beam
splitter in a first stage is formed at the central part of the
rectangular PLC chip and a second and a third polarization beam splitters
in a second stage are formed in parallel sandwiching the polarization
beam splitter in the first stage in between; and one of the first and
second 90-degree hybrid circuits is formed on the opposite side of the
polarization beam splitter in the first stage with respect to the second
polarization beam splitter and the other of the first and second
90-degree hybrid circuits is formed on the opposite side of the
polarization beam splitter in the first stage with respect to the third
polarization beam splitter.

8. The PLC-type demodulator according to claim 7, wherein an output end
of the polarization beam splitter in the first stage and an input end of
the second polarization beam splitter are connected from the output end
toward the input end, using bend waveguides with the absolute value of
the total of the rotation angles the sign of which is not reversed being
greater than 180 degrees, as a folded waveguide, and the output end of
the polarization beam splitter in the first stage and an input end of the
third polarization beam splitter are connected from the output end toward
the input end, using bend waveguides with the absolute value of the total
of the rotation angles the sign of which is not reversed being greater
than 180 degrees, as a folded waveguide.

9. The PLC-type demodulator according to claim 7, wherein an output end
of the second polarization beam splitter and an input end of one of the
first and second 90-degree hybrid circuits are connected from the output
end toward the input end, using bend waveguides with the absolute value
of the total of the rotation angles the sign of which is not reversed
being greater than 180 degrees, as a first folded waveguide, and an
output end of the third polarization beam splitter and an input end of
the other of the first and second 90-degree hybrid circuits are connected
from the output end toward the input end, using bend waveguides with the
absolute value of the total of the rotation angles the sign of which is
not reversed being greater than 180 degrees, as a second folded
waveguide.

10. The PLC-type demodulator according to claim 1, wherein the
polarization beam splitter is a Mach-Zehnder interferometer comprising an
input side coupler as an input end of the polarization beam splitter, an
output side coupler as an output end of the polarization beam splitter,
and two arm waveguides connected between both the couplers.

11. The PLC-type demodulator according to claim 9, wherein each of the
polarization beam splitter, the second polarization beam splitter, and
the third polarization beam splitter is a Mach-Zehnder interferometer
comprising an input side coupler as an input end of the polarization beam
splitter, an output side coupler as an output end of the polarization
beam splitter, and two arm waveguides connected between both the
couplers, a cross port of the output side coupler of the second
polarization beam splitter and an input side coupler of one of the first
and second 90-degree hybrid circuits are connected by the first folded
waveguide, and a cross port of the output side coupler of the third
polarization beam splitter and an input side coupler of the other of the
first and second 90-degree hybrid circuits are connected by the second
folded waveguide.

12. The PLC-type demodulator according to claim 1, wherein the second
input port has an input port of X-polarization local oscillation light
having the same polarized wave and the same wavelength as the
X-polarization coherent modulated signal and an input port of
Y-polarization local oscillation light having the same polarized wave and
the same wavelength as the Y-polarization coherent modulated signal.

13. The PLC-type demodulator according to claim 1, further comprising: a
first path through which the X-polarization coherent modulated signal
split in the polarization beam splitter propagates and which connects the
polarization beam splitter and the first 90-degree hybrid circuit; a
second path through which the Y-polarization coherent modulated signal
split in the polarization beam splitter propagates and which connects the
polarization beam splitter and the second 90-degree hybrid circuit; and a
half-wavelength plate inserted into the first path or the second path,
wherein the PLC-type demodulator is configured so that signals enter the
first and second 90-degree hybrid circuits, respectively, in the same
polarization state.

14. The PLC-type demodulator according to claim 13, wherein the number of
the second input ports is one, and the PLC-type demodulator further
comprises a path which is configured so as to split X-polarization or
Y-polarization local oscillation light input from the second input port
within the planar lightwave circuit and input it to the first and second
90-degree hybrid circuits, respectively.

15. The PLC-type demodulator according to claim 7, further comprising:
two inspection input ports for inputting light caused to pass through
only the second and third polarization beam splitters; and two inspection
output ports for outputting light having passed through the second and
third polarization beam splitters, respectively, wherein a heater is
provided on at least one of the two arm waveguides of the polarization
beam splitter in the first stage.

16. An optical transmission system that uses a PLC-type demodulator, the
system comprising: a transmitter that modulates a lightwave and outputs a
polarization-multiplexed light signal, an optical transmission path that
transmits the polarization-multiplexed light signal output from the
transmitter; and a receiver that performs coherent reception of the
polarization-multiplexed light signal transmitted through the
transmission path, wherein the receiver includes: a light source that
outputs local oscillation light; the PLC-type demodulator according to
claim 1; an optical detector for X-polarization I channel and Q channel;
an optical detector for Y-polarization I channel and Q channel; and a
digital signal processing circuit.

17. The optical transmission system according to claim 16, wherein a
method of the modulation is quadrature phase shift keying.

Description:

CROSS-REFERENCES TO RELATED APPLICATIONS

[0001] This application is a continuation application of International
Application No. PCT/JP2010/065313, filed Sep. 7, 2010, which claims the
benefit of Japanese Patent Application No. 2009-206115, filed Sep. 7,
2009. The contents of the aforementioned applications are incorporated
herein by reference in their entities.

TECHNICAL FIELD

[0002] The present invention relates to a PLC-type demodulator that
receives a polarization-multiplexed coherent modulated signal, which is
obtained by performing coherent modulation on each of X-polarization
light and Y-polarization light and then polarization multiplexing them,
and an optical transmission system using the same.

BACKGROUND ART

[0003] As a modulation system of large-capacity signal transmission, a
polarization-multiplexed digital coherent transmission system is
promising, in which X-polarization light and Y-polarization light are
each modulated and then polarization-multiplexed, and demodulated by a
digital coherent receiver. Here, the coherent transmission system is a
modulation system, in which a phase of light or even an amplitude in
addition to the phase of light is modulated on the transmission side and
on the reception side, local oscillation light (LO light) and signal
light after transmission are mixed using an interference circuit, called
a 90-degree hybrid, and received by a balanced photo detector (B-PD), and
thereby, the signal light is demodulated by splitting the signal light
into the real number part and the imaginary number part when the electric
field of light is regarded as a complex number. However, in the receiver,
the input polarization-multiplexed signal is polarization-split
optically, but in general, the base polarization state of a polarization
beam splitter (PBS) used therein and the base polarization state of the
polarization-multiplexed signal light do not coincide with each other,
and therefore, two orthogonal polarization components output from the PBS
will not form the signal light the polarization multiplexing of which is
demultiplexed.

[0004] However, by performing digital signal processing on an electric
signal output from the B-PD as to the respective components
polarization-multiplexed optically, it is possible to perform
polarization demultiplexing. Further, by the digital signal processing,
it is also possible to estimate the relative phase difference between the
signal light and LO light and to perform processing, such as dispersion
compensation and error correction.

[0005] As described above, the system that considerably simplifies optical
processing by performing digital signal processing on a
polarization-multiplexed coherent modulated signal in the receiver and
further improves reception characteristics also is called a
polarization-multiplexed digital coherent transmission system and very
promising.

[0006] As a typical and practical method of the coherent modulation
system, the spread of a quadrature phase shift keying is being encouraged
and the quadrature phase shift keying by polarization multiplexing is
known as a DP-QPSK (Dual Polarization Quadrature Phase Shift Keying).
With the DP-QPSK modulation system, when the symbol rate is 10 GSymbol/s,
the bit rate is 40 Gbit/s and when the symbol rate is 25 GSymbol/s, the
bit rate is 100 Gbit/s, and therefore, it is possible to improve
frequency use efficiency. The DP-QPSK modulation system when simply
referred to means a system that applies a digital coherent receiver at
the time of demodulation.

[0007] In the DP-QPSK modulation system demodulator (DP-QPSK demodulator),
first, a DP-QPSK signal, which is a multiplexed signal of an
X-polarization QPSK signal and a Y-polarization QPSK signal, is split
into an X-polarization QPSK signal and a Y-polarization QPSK signal by a
polarization beam splitter (PBS). Further, by an X-polarization 90-degree
hybrid and a Y-polarization 90-degree hybrid, the split X-polarization
QPSK signal and Y-polarization QPSK signal, and local oscillation light
(LO light) are mixed, respectively. By receiving this mixed light with a
B-PD combined together, each polarization phase-modulated signal (QPSK
signal light) is converted into an intensity-modulated signal and
components corresponding to the real part and the imaginary part
(I-component and Q-component) of the electric field of the signal light
in each polarization are extracted independently. In general, the
90-degree hybrid is known as a circuit that branches the input signal
light and local oscillation light into two, respectively, gives a phase
difference of 90 degrees to the local oscillation light branched into two
as a relative phase difference of lightwave, and then mixes one of the
signal light branched into two and one of the local oscillation light
into two, and the other of the signal light branched into two and the
other of the local oscillation light into two, respectively.

[0008] The PBS and the 90-degree hybrid are realized individually by a
space optical system or quartz-based planar lightwave circuit (PLC) as
prior art (see Non-Patent Documents 1 to 6).

[0009] In Non-Patent Documents 1 to 3, the 90-degree hybrid having a
configuration in which a coupler and a PBS are combined on one PLC is
disclosed and the technique to reduce the time difference (skew) between
I- and Q-components by making the same the optical waveguide lengths
between the I- and Q-components, respectively is disclosed.

[0017] When configuring a DP-QPSK demodulator that receives a DP-QPSK
signal and extracts independently the I-component and the Q-component of
each polarization component by combining the PBS and the 90-degree hybrid
formed in each individual device as disclosed in Non-Patent Documents 1
to 6, problems as follows occur.

[0018] The DP-QPSK demodulator is configured to include a PBS and a
90-degree hybrid formed separately from the PBS (that is, formed on a
different chip), and therefore, it is necessary to optically connect one
device on which the PBS is formed (for example, PLC) and another device
on which the 90-degree hybrid is formed (for example, PLC). Because of
that, connection loss occurs and at the same time, alignment work and
bonding work for the optical connection thereof are necessary, resulting
in an increase in the number of processes and in the manufacturing cost.

[0019] The present invention has been made by focusing attention on the
conventional problems described above and an object thereof is to provide
a PLC-type demodulator that reduces the connection loss between the
polarization beam splitter and the 90-degree hybrid circuit and aims at
reducing the manufacturing cost, and an optical transmission system using
the same.

[0020] Further, the inventors of the present invention have discovered
such a problem that if the PBS and the 90-degree hybrid are formed on one
PLC, the skew between the X- and Y-polarization components increases as a
result of intensive research, and have invented a configuration to solve
the problem.

[0021] When an X-polarization QPSK signal and a Y-polarization QPSK signal
split in the PBS propagates respectively through the PBS in the second
stage or X-polarization and Y-polarization 90-degree hybrid circuits,
respectively, if there is a difference in path length, there is produced
a difference between the times at which these signals are output. It is
desirable for this time difference (skew) to be one-hundredth or less of,
for example, a symbol time interval defined by an inverse of the symbol
rate and to be 1 ps or less for a signal with 10 GSymbol/s. In order to
reduce the skew to 1 ps or less, it is necessary to reduce the optical
path length difference to about 300 mm in a vacuum or to about 200 mm or
less in silica glass with a refractive index of about 1.5, and it is hard
to adjust this precision when a space optical system is used or when
components are connected by optical fibers.

[0022] Because of the above, another object of the present invention is to
provide a high-performance PLC-type demodulator that reduces the skew
between the X- and Y-polarization components and an optical transmission
system using the same.

[0023] A first aspect of the present invention is a PLC-type demodulator
that receives and demodulates a polarization-multiplexed coherent
modulated signal, the demodulator comprising: one PLC chip in which a
planar lightwave circuit is formed; a first input port provided at an
input end of the PLC chip and inputting the polarization-multiplexed
coherent modulated signal into the planar lightwave circuit; a second
input port provided at the input end of the PLC chip and inputting local
oscillation light into the planar lightwave circuit; at least one
polarization beam splitter that splits the polarization-multiplexed
coherent modulated signal input from the first input port into an
X-polarization coherent modulated signal and a Y-polarization coherent
modulated signal; a first 90-degree hybrid circuit that mixes and outputs
the X-polarization coherent modulated signal and the local oscillation
light input from the second input port; and a second 90-degree hybrid
circuit that mixes and outputs the Y-polarization coherent modulated
signal and the local oscillation light input from the second input port,
wherein the at least one polarization beam splitter, the first 90-degree
hybrid circuit, and the second 90-degree hybrid circuit are integrated
within the planar lightwave circuit.

[0024] According to this configuration, alignment work and bonding work
for the optical connection of the polarization beam splitter and the two
90-degree hybrid circuits are no longer necessary, and therefore, it is
possible to eliminate the connection loss between the polarization beam
splitter and the two 90-degree hybrid circuits and to reduce the
manufacturing cost.

[0025] A second aspect of the present invention is the PLC-type
demodulator in the first aspect of the present invention, further
comprising a second polarization beam splitter that splits the mixed
light of the X-polarization local oscillation light and the
Y-polarization local oscillation light into the X-polarization local
oscillation light and the Y-polarization local oscillation light, wherein
the polarization beam splitter and the second polarization beam splitter
each have an input side coupler and an output side coupler; the
polarization beam splitter and the second polarization beam splitter are
provided so that the input side coupler is located on the output end side
of the PLC chip opposite to the input end and the output side coupler is
located on the input end side; and the first 90-degree hybrid circuit,
the polarization beam splitter, the second polarization beam splitter,
and the second 90-degree hybrid circuit are arranged in this order in a
direction perpendicular to the direction going from the input end toward
the output end, wherein the PLC-type demodulator further comprises: a
waveguide that connects the first input port and the input side coupler
of the polarization beam splitter and has a bent region so as to fold
propagating light; a waveguide that connects the second input port and
the input side coupler of the second polarization beam splitter and has a
bent region so as to fold propagating light; a first waveguide that
connects the output side coupler of the polarization beam splitter and
the first 90-degree hybrid circuit, transmits one of the X-polarization
coherent modulated signal and the Y-polarization coherent modulated
signal, and has a bent region so as to fold propagating light; a second
waveguide that connects the output side coupler of the polarization beam
splitter and the second 90-degree hybrid circuit, transmits the other of
the X-polarization coherent modulated signal and the Y-polarization
coherent modulated signal, and has a bent region so as to fold
propagating light; a third waveguide that connects the output side
coupler of the second polarization beam splitter and the first 90-degree
hybrid circuit, transmits one of the X-polarization local oscillation
light and the Y-polarization local oscillation light, and has a bent
region so as to fold propagating light; and a fourth waveguide that
connects the output side coupler of the second polarization beam splitter
and the second 90-degree hybrid circuit, transmits the other of the
X-polarization local oscillation light and the Y-polarization local
oscillation light, and has a bent region so as to fold propagating light,
and wherein the optical path length of the first waveguide and the
optical path length of the second waveguide are the same.

[0026] A third aspect of the present invention is the PLC-type demodulator
in the second aspect of the present invention, wherein the optical path
lengths of the first waveguide, the second waveguide, the third
waveguide, and the fourth waveguide are the same; and the second
waveguide and the third waveguide intersect with each other at an
intersection angle 2θ, wherein the first waveguide has a first bend
waveguide connected to the output side coupler of the polarization beam
splitter, a first straight waveguide connected to the first bend
waveguide, and a second bend waveguide connected to the first straight
waveguide; the second waveguide has a third bend waveguide connected to
the output side coupler of the polarization beam splitter, a second
straight waveguide connected to the third bend waveguide, and a fourth
bend waveguide connected to the second straight waveguide; the third
waveguide has a fifth bend waveguide connected to the output side coupler
of the second polarization beam splitter, a third straight waveguide
connected to the fifth bend waveguide, and a sixth bend waveguide
connected to the third straight waveguide; and the fourth waveguide has a
seventh bend waveguide connected to the output side coupler of the second
polarization beam splitter, a fourth straight waveguide connected to the
seventh bend waveguide, and an eighth bend waveguide connected to the
fourth straight waveguide, wherein the first, third, fifth and seventh
bend waveguides have the same shape as a fan-shaped arc with a bend
radius r and a central angle θ; and the second, fourth, sixth and
eighth bend waveguides have the same shape as a fan-shaped arc with the
bend radius r and a central angle greater than π-2θ
(0<θ<π/2), and wherein a length l of the first, second,
third and fourth straight waveguides satisfies a relationship 1=(2r cos
θ-r-p/2)/sin θ when an interval between two waveguides in the
proximity of the output side coupler is assumed to be p; and the second
waveguide and the third waveguide intersect with each other at a boundary
between the second straight waveguide and the fourth bend waveguide and
at a boundary between the third straight waveguide and the fifth bend
waveguide.

[0027] A fourth aspect of the present invention is the PLC-type
demodulator in the first aspect of the present invention, wherein a path
through which the X-polarization coherent modulated signal propagates and
a path through which the Y-polarization coherent modulated signal
propagates are set so that all the effective optical path lengths from
the input end toward the output end of the PLC chip are the same.

[0028] If there is a difference between the path lengths of the optical
waveguides through which the X-polarization coherent modulated signal
(for example, the QPSK signal (X-signal)) and the Y-polarization coherent
modulated signal (for example, the QPSK signal (Y-signal)) split in the
polarization beam splitter propagate, respectively, there is produced a
difference between the times at which those signals are output. In the
fourth aspect, the path through which the X-signal propagates and the
path through which the Y-signal propagates are set so that all the
effective optical path lengths from the input end to the output end are
the same, and therefore, it is possible to realize a high-performance
PLC-type demodulator that reduces the skew between the X- and
Y-polarization components.

[0029] A fifth aspect of the present invention is the PLC-type demodulator
in the first aspect of the present invention, wherein the number of the
polarization beam splitters is two or more, and the polarization beam
splitters and the first and second 90-degree hybrid circuits are
arranged, respectively, in the proximity of each other.

[0030] According to this configuration, by arranging the polarization beam
splitters and the two 90-degree hybrid circuits, respectively, in the
proximity of each other, the difference in the effective optical path
lengths of the signal light that propagates through a plurality of paths
is suppressed from occurring, and therefore, the skew between the signal
lights that pass through different paths is reduced.

[0031] A sixth aspect of the present invention is the PLC-type demodulator
in the fifth aspect of the present invention, wherein the polarization
beam splitters are cascade-connected in two or more stages.

[0032] According to this configuration, it is possible to increase the
extinction ratio of the polarization beam splitter.

[0033] A seventh aspect of the present invention is the PLC-type
demodulator in the first aspect of the present invention, wherein the PLC
chip is rectangular, substantially close to square, in shape; a
polarization beam splitter in a first stage is formed at the central part
of the rectangular PLC chip and a second and a third polarization beam
splitters in a second stage are formed in parallel sandwiching the
polarization beam splitter in the first stage in between; and one of the
first and second 90-degree hybrid circuits is formed on the opposite side
of the polarization beam splitter in the first stage with respect to the
second polarization beam splitter and the other of the first and second
90-degree hybrid circuits is formed on the opposite side of the
polarization beam splitter in the first stage with respect to the third
polarization beam splitter.

[0034] According to this configuration, it is possible to make an attempt
to downsize the PLC chip and to realize a compact PLC-type demodulator.

[0035] An eighth aspect of the present invention is the PLC-type
demodulator in the seventh aspect of the present invention, wherein an
output end of the polarization beam splitter in the first stage and an
input end of the second polarization beam splitter are connected from the
output end toward the input end, using bend waveguides with the absolute
value of the total of the rotation angles the sign of which is not
reversed being greater than 180 degrees, as a folded waveguide, and the
output end of the polarization beam splitter in the first stage and an
input end of the third polarization beam splitter are connected from the
output end toward the input end, using bend waveguides with the absolute
value of the total of the rotation angles the sign of which is not
reversed being greater than 180 degrees, as a folded waveguide.

[0036] According to this configuration, it is possible to arrange the two
or more polarization beam splitters and the two 90-degree hybrid circuits
within one PLC chip in parallel and in the proximity of each other while
securing a bend radius in such a degree that the loss due to leaked light
is not problematic for each bend waveguide. Consequently, it is possible
to realize a compact PLC-type demodulator without deteriorating optical
characteristics.

[0037] A ninth aspect of the present invention is the PLC-type demodulator
in the seventh aspect of the present invention, wherein an output end of
the second polarization beam splitter and an input end of one of the
first and second 90-degree hybrid circuits are connected from the output
end toward the input end, using bend waveguides with the absolute value
of the total of the rotation angles the sign of which is not reversed
being greater than 180 degrees, as a first folded waveguide, and an
output end of the third polarization beam splitter and an input end of
the other of the first and second 90-degree hybrid circuits are connected
from the output end toward the input end, using bend waveguides with the
absolute value of the total of the rotation angles the sign of which is
not reversed being greater than 180 degrees, as a second folded
waveguide.

[0038] According to this configuration, it is possible to arrange the two
or more polarization beam splitters and the two 90-degree hybrid circuits
within one PLC chip in parallel and in the proximity of each other while
securing a bend radius in such a degree that the loss due to leaked light
is not problematic for each bend waveguide. Consequently, it is possible
to realize a compact PLC-type demodulator without deteriorating optical
characteristics.

[0039] A tenth aspect of the present invention is the PLC-type demodulator
in the first aspect of the present invention, wherein the polarization
beam splitter is a Mach-Zehnder interferometer comprising an input side
coupler as an input end of the polarization beam splitter, an output side
coupler as an output end of the polarization beam splitter, and two arm
waveguides connected between both the couplers.

[0040] An eleventh aspect of the present invention is the PLC-type
demodulator in the ninth aspect of the present invention, wherein each of
the polarization beam splitter, the second polarization beam splitter,
and the third polarization beam splitter is a Mach-Zehnder interferometer
comprising an input side coupler as an input end of the polarization beam
splitter, an output side coupler as an output end of the polarization
beam splitter, and two arm waveguides connected between both the
couplers, a cross port of the output side coupler of the second
polarization beam splitter and an input side coupler of one of the first
and second 90-degree hybrid circuits are connected by the first folded
waveguide, and a cross port of the output side coupler of the third
polarization beam splitter and an input side coupler of the other of the
first and second 90-degree hybrid circuits are connected by the second
folded waveguide.

[0041] According to this configuration, it is possible to increase the
bend radius of the folded waveguide that connects the second polarization
beam splitter and one of the 90-degree hybrid circuits and the bend
radius of the folded waveguide that connects the third polarization beam
splitter and the other of the 90-degree hybrid circuits, respectively.
Further, it can be expected to increase the polarization extinction ratio
of each of the polarization beam splitters.

[0042] A twelfth aspect of the present invention is the PLC-type
demodulator in the first aspect of the present invention, wherein the
second input port has an input port of X-polarization local oscillation
light having the same polarized wave and the same wavelength as the
X-polarization coherent modulated signal and an input port of
Y-polarization local oscillation light having the same polarized wave and
the same wavelength as the Y-polarization coherent modulated signal.

[0043] According to this configuration, it is possible to realize a
high-performance PLC-type receiver that reduces the skew between two
signals which are in the same polarization state and enter the two
90-degree hybrid circuits, respectively.

[0044] A thirteenth aspect of the present invention is the PLC-type
demodulator in the first aspect of the present invention, further
comprising: a first path through which the X-polarization coherent
modulated signal split in the polarization beam splitter propagates and
which connects the polarization beam splitter and the first 90-degree
hybrid circuit; a second path through which the Y-polarization coherent
modulated signal split in the polarization beam splitter propagates and
which connects the polarization beam splitter and the second 90-degree
hybrid circuit; and a half-wavelength plate inserted into the first path
or the second path, wherein the PLC-type demodulator is configured so
that signals enter the first and second 90-degree hybrid circuits,
respectively, in the same polarization state.

[0045] There exists a difference in the effective refractive index caused
by birefringence between the X-polarization coherent modulated signal
(for example, the QPSK signal (X-signal)) and the Y-polarization coherent
modulated signal (for example, the QPSK signal (Y-signal)) split in the
polarization beam splitter, and therefore, this forms a factor of the
skew.

[0046] According to this configuration, it is possible to reduce the skew
(time difference) that occurs due to the effective refractive index
difference resulting from birefringence between the signals that enter
each of the 90-degree hybrid circuits because the X-signal and the
Y-signal enter the two 90-degree hybrid circuits in the same polarization
state, respectively. Due to this, it is possible to realize a
high-performance PLC-type demodulator that reduces the skew between the
two signals.

[0047] A fourteenth aspect of the present invention is the PLC-type
demodulator in the thirteenth aspect of the present invention, wherein
the number of the second input ports is one, and the PLC-type demodulator
further comprises a path which is configured so as to split
X-polarization or Y-polarization local oscillation light input from the
second input port within the planar lightwave circuit and input it to the
first and second 90-degree hybrid circuits, respectively.

[0048] According to this configuration, it is necessary to provide only
one LO light source that outputs X-polarization or Y-polarization LO
light as a light source of local oscillation light (LO light source), and
therefore, it is possible to further reduce the manufacturing cost of the
receiver.

[0049] A fifteenth aspect of the present invention is the PLC-type
demodulator in the seventeenth aspect of the present invention, further
comprising: two inspection input ports for inputting light caused to pass
through only the second and third polarization beam splitters; and two
inspection output ports for outputting light having passed through the
second and third polarization beam splitters, respectively, wherein a
heater is provided on at least one of the two arm waveguides of the
polarization beam splitter in the first stage.

[0050] According to this configuration, while measuring light output from
the two inspection output ports, a voltage is applied to the heater and
the phase trimming is performed individually so that the polarization
extinction ratios of the second and third polarization beam splitters
corresponding to the output ports, respectively, satisfy desired values.
Due to this, it is possible to adjust the polarization extinction ratios
of the second and third polarization beam splitters to desired values.

[0051] A sixteenth aspect of the present invention is an optical
transmission system that uses a PLC-type demodulator, the system
comprising: a transmitter that modulates a lightwave and outputs a
polarization-multiplexed light signal, an optical transmission path that
transmits the polarization-multiplexed light signal output from the
transmitter; and a receiver that performs coherent reception of the
polarization-multiplexed light signal transmitted through the
transmission path, wherein the receiver includes: a light source that
outputs local oscillation light; the PLC-type demodulator according to
the first aspect of the present invention; an optical detector for
X-polarization I channel and Q channel; an optical detector for
Y-polarization I channel and Q channel; and a digital signal processing
circuit.

[0052] A seventeen aspect of the present invention is the optical
transmission system in the sixteenth aspect of the invention, wherein a
method of the modulation is quadrature phase shift keying.

[0053] According to an aspect of the present invention, it is possible to
realize a PLC-type demodulator that eliminates the connection loss
between the polarization beam splitter and the 90-degree hybrid circuit
and aims at reducing the manufacturing cost, and an optical transmission
system using the same.

[0054] Further, according to another aspect of the present invention, it
is possible to realize a high-performance PLC-type demodulator that
reduces the skew between X- and Y-polarization components, and an optical
transmission system using the same.

BRIEF DESCRIPTION OF THE DRAWINGS

[0055] FIG. 1 is a block diagram showing a basic configuration of a
PLC-type DP-QPSK demodulator according a first embodiment of the present
invention.

[0056]FIG. 2 is an outline configuration diagram showing a polarization
beam splitter (PBS) used in the PLC-type DP-QPSK demodulator in FIG. 1.

[0057]FIG. 3 is an outline configuration diagram showing a 90-degree
hybrid circuit used in the DP-QPSK demodulator in FIG. 1.

[0069] FIG. 15 is an explanatory diagram showing details of another folded
waveguide in the demodulator shown in FIG. 9.

[0070] FIG. 16 is an explanatory diagram showing details of another folded
waveguide in the demodulator shown in FIG. 9.

[0071]FIG. 17 is an explanatory diagram showing details of another folded
waveguide in the demodulator shown in FIG. 9.

[0072]FIG. 18 is an explanatory diagram showing details of another folded
waveguide in the demodulator shown in FIG. 9.

[0073]FIG. 19 is a block diagram showing a basic configuration of a
PLC-type DP-QPSK demodulator according to a sixth embodiment of the
present invention.

[0074]FIG. 20 is a block diagram showing an outline configuration of an
optical transmission system using a PLC-type DP-QPSK demodulator
according to an embodiment of the present invention.

[0075] FIG. 21 is a block diagram showing a basic configuration of a
PLC-type DP-QPSK demodulator according to a seventh embodiment of the
present invention.

[0076] FIG. 22 is a diagram for explaining a configuration of a second
folded region of the PLC-type DP-QPSK demodulator shown in FIG. 21.

[0077] FIG. 23 is a block diagram showing a basic configuration of a
PLC-type DP-QPSK demodulator according to an eighth embodiment of the
present invention.

[0078] FIG. 24 is a block diagram showing a basic configuration of a
PLC-type DP-QPSK demodulator according to the eighth embodiment of the
present invention.

DESCRIPTION OF EMBODIMENTS

[0079] Hereinafter, embodiments that embody the present invention are
explained based on the drawings. In the explanation of each of the
embodiments, the same reference numeral is attached to the same component
and duplicated explanation is omitted.

[0080] <PLC-Type DP-QPSK Demodulator>

[0081] First, as an example of a PLC-type demodulator that receives a
polarization-multiplexed coherent modulated signal, each embodiment of a
DP-QPSK modulation system PLC-type DP-QPSK demodulator that receives a
DP-QPSK signal is explained.

[0082] (PLC-Type DP-QPSK Demodulator According to First Embodiment)

[0083] a PLC-type DP-QPSK demodulator 1 according to a first embodiment is
explained based on FIG. 1 to FIG. 5.

[0084] The PLC-type DP-QPSK demodulator 1 is a DP-QPSK modulation system
demodulator that receives, from a transmitter (not shown schematically),
a DP-QPSK signal 2 formed by multiplexing an X-polarization QPSK signal
and a Y-polarizing QPSK signal, which are obtained by performing
quadrature phase shift keying on X-polarization light and Y-polarization
light orthogonal to each other, respectively.

[0085] In the present specification, the "DP-QPSK demodulator" used in the
DP-QPSK modulation system optical transmission system means a device to
which a DP-QPSK signal (polarization-multiplexed quadrature phase shift
keyed signal) formed by multiplexing an X-polarization QPSK signal and a
Y-polarization QPSK signal is input, and which splits the signal into two
orthogonal polarization components by a polarization beam splitter (PBS)
and then mixes the signal light of each polarization component and local
oscillation light (LO light) by an interference circuit called a
90-degree hybrid and outputs it to a balanced photodiode (B-PD). That is,
the "DP-QPSK demodulator" referred to in the present specification is a
receiver used in the DP-QPSK modulation system optical transmission
system not including a B-PD.

[0086] The demodulator of the present invention is a demodulator including
at least a PBS and a 90-degree hybrid and its application is not limited
to a DP-QPSK modulated signal and it can also be applied to an optical
transmission system that uses a general coherent modulation system, such
as QAM (Quadrature Amplitude Modulation) and OFDM (Orthogonal Frequency
Division Multiplexing). Hereinafter, description is given on the
supposition that the present invention is applied to the DP-QPSK
modulation.

[0087] The PLC-type DP-QPSK demodulator (hereinafter, referred to as
demodulator) 1 includes one PLC chip 3 in which a planar lightwave
circuit is formed. In the PLC chip 3, a planar lightwave circuit (PLC)
including a plurality of optical waveguides comprising a core and
cladding by combining the optical fiber manufacturing technique and the
semiconductor micro fabrication technique is formed on a substrate, not
shown schematically, such as quartz substrate and silicon substrate. The
PLC is, for example, a quartz-based planar lightwave circuit.

[0088] At an input end 4 of the PLC chip 3, an input port In1 of the
DP-QPSK signal 2 as a polarization-multiplexed coherent modulated signal
and input ports In2, In3 of local oscillation light are provided. To the
input port In2, local oscillation light (X-polarization LO light) having
the same polarized wave and the same wavelength as the X-polarization
QPSK signal is input. To the input port In3, local oscillation light
(Y-polarization LO light) having the same polarized wave and the same
wavelength as the Y-polarization QPSK signal is input.

[0089] At an output end 5 of the PLC chip 3, output ports Out1 to Out8 of
signal light are provided, respectively. From the output ports Out1, 2,
signal light of the I-channel component (real part on the complex plane:
cosine component) of the orthogonal components I, Q of the X-polarization
QPSK signal converted into an intensity-modulated signal is output and
from the output ports Out3, 4, signal light of the Q-channel component
(imaginary part on the complex plane: sine component) of the orthogonal
components I, Q of the X-polarization QPSK signal converted into an
intensity-modulated signal is output, respectively.

[0090] From the output ports Out5, 6, signal light of the I-channel
component of the Y-polarization QPSK signal converted into an
intensity-modulated signal is output and from the output ports Out7, 8,
signal light of the Q-channel of the Y-polarization QPSK signal converted
into an intensity-modulated signal is output, respectively.

[0091] Within the PLC of the PLC chip 3, there are integrated a
polarization beam splitter (PBS) 30 that polarization-splits the DP-QPSK
signal 2 into the X-polarization QPSK signal (X-signal) and the
Y-polarization QPSK signal (Y-signal), and two X-polarization and
Y-polarization 90-degree hybrid circuits 41, 42.

[0092] As shown in FIG. 2, the PBS 30 includes a Mach-Zehnder
interferometer (MZI) 35 having two couplers 31, 32 and two arm waveguides
33, 34 connected between both the couplers 31, 32. The coupler 31 on the
input side and the coupler 32 on the output side are each a 3-dB coupler
including a directional coupler (DC), respectively. To one of two input
ports of the coupler 31 on the input side, an input optical waveguide 7
is connected (see FIG. 1, FIG. 2). That is, the PBS 30 is configured to
split the DP-QPSK signal 2 as a polarization-multiplexed coherent
modulated signal into the X-polarization component and the Y-polarization
component.

[0093] Next, the 90-degree hybrid circuits 41, 42 are explained.

[0094] The 90-degree hybrid circuit 41 mixes the X-polarization QPSK
signal as the X-polarization signal split in the PBS 30 and local
oscillation light, splits the X-polarization QPSK signal into the
orthogonal components I, Q, and outputs them. That is, the 90-degree
hybrid 41 mixes and outputs the X-polarization signal split in the PBS 30
and local oscillation signal. As shown in FIG. 3, the 90-degree hybrid
circuit 41 includes two input side couplers 20, 21, two output side
couplers 22, 23, and arm waveguides 24 to 27 connected between the input
side couplers 20, 21 and the output side couplers 22, 23. As the input
side couplers 20, 21, a Y-branch coupler is used, respectively, and as
the output side couplers 22, 23, a wavelength insensitive directional
coupler (WINC) is used, respectively. In the following explanation, the
arm waveguides 24 to 27 are sometimes referred to as path 0 to path 3.

[0095] On an input port 41a of the 90-degree hybrid circuit 41, the
X-signal (X-polarization QPSK signal) is incident and on an input port
41b, X-polarization LO light is incident, respectively, (see FIG. 1, FIG.
3). The 90-degree hybrid circuit 41 is configured so that the X-signal,
after being branched into two in the input side coupler 20, passes
through the paths 0, 2, respectively, and is incident on one of the input
ports of the couplers 22, 23. The 90-degree hybrid circuit 41 is
configured so that the LO light, after being branched into two in the
input side coupler 21, passes through the paths 1, 3, respectively, and
is incident on the other input port of the couplers 22, 23.

[0096] The paths 0, 2 are set so as to have an identical optical path
length L and the paths 1, 3 are set so that the difference in their
optical path lengths is 90 degrees in terms of phase. For example, it may
also be possible to lengthen the optical path length of the path 1 longer
than the optical path length of the paths 0 and 2 by an amount
corresponding to π/4 radian in terms of phase and to shorten the
optical path length of the path 3 shorter than the optical path length of
the paths 0 and 2 by an amount corresponding to π/4 radian in terms of
length.

[0097] As shown in FIG. 4, the path 0 (the arm waveguide 24) includes a
bend waveguide 24a of a rotation angle +θ1, a straight
waveguide 24b of a length l1, a bend waveguide 24c of a rotation
angle -θ1, a straight waveguide 24d of a length h, a bend
waveguide 24e of a rotation angle -θ2, a straight waveguide
24f of a length l2, and a bend waveguide 24g of a rotation angle
+θ2. A bend radius r of each bend waveguide is set to an
optimum value, for example, 2,000 μm. In FIG. 4, reference numeral 24h
represents a section in which a phase trimming heater 28 is disposed.

[0098] In discussing the value of a rotation angle θ of the bend
waveguide, it is defined that the sign is plus when the bend waveguide
rotates counterclockwise in the direction in which light travels and
minus when it rotates clockwise in the direction in which light travels.
The rotation angle θ is an angle (central angle) formed by two bend
radii that form an arc when the bend waveguide coincides with the arc of
the bend radius r (curvature radius r). Consequently, the fan-shaped arc
of the bend radius r and the central angle (that is, the rotation angle)
θ is the shape of the bend waveguide of the rotation angle θ.

[0099] The 90-degree hybrid circuit 41 has a structure in which the
optical path length of the path 0 can be adjusted to an arbitrary optical
path length by adjusting the four parameters θ1,
θ2, l1, l2 under the condition that the bend radius
r of each of the bend waveguides 24a, 24c, 24e, 24g is fixed. Other paths
1 to 3 (the arm waveguides 25 to 27) also have the same structure.

[0100] In the 90-degree hybrid circuit 41, a pitch Pi between input ports
41a, 41b (pitch between input ports) is determined uniquely by the
following expression using an angle (intersection angle) α at which
the path 1 and the path 2 intersect with each other, the bend radius r of
the bend waveguide 24a, and a pitch Py (see FIG. 5) between the output
waveguides of the Y-branch couplers 20, 21.

Pi=2r(1-cos α)+Py

[0101] Here, the bend waveguides of the arm waveguides 24 to 27 have the
same bend radius r.

[0102] The 90-degree hybrid circuit 42 also has the same structure as that
of the 90-degree hybrid circuit 41. In the demodulator 1 having the above
configuration, first, the DP-QPSK signal 2 input from the input port In1
passes through the input optical waveguide 7 and enters the PBS 30 and is
polarization-split into the X-polarization QPSK signal (X-signal) and the
Y-polarization QPSK signal (Y-signal) by the PBS 30. The X-signal passes
through an optical waveguide 8 and enters the input port 41a of the
90-degree hybrid circuit and the Y-signal passes through an optical
waveguide 9 and enters the input port 42a of the 90-degree hybrid circuit
42, respectively.

[0103] In the 90-degree hybrid circuit 41, the X-polarization LO light
that passes through an optical waveguide 10 from the input port In2 and
enters the circuit and the X-signal that passes through the optical
waveguide 8 and enters the circuit are mixed. That is, the X-signal that
passes through the path 0 and the LO light that passes through the path 1
are mixed in the coupler 22 on the output side and at the same time, the
X-signal that passes through the path 2 and the LO light that passes
through the path 3 are mixed in the coupler 23 on the output side.

[0104] Due to this, the signal light of the I-channel component and the
signal light of the Q-channel component, which is the X-signal, that is,
the X-polarization QPSK signal, converted into an intensity-modulated
signal, are extracted independently. The signal light of the I-channel
component in X polarization passes through output optical waveguides 11,
12 and is output from the output ports Out1, 2. On the other hand, the
signal light of the Q-channel component passes through output optical
waveguides 13, 14 and is output from the output ports Out3, 4,
respectively. The signal light of the I-channel component and the
Q-channel component in X polarization is input to balanced photodiodes
(B-PD) 61 and 62, respectively, via, for example, an optical fiber or
without passing therethrough.

[0105] On the other hand, in the 90-degree hybrid circuit 42, the
Y-polarization LO light that passes through an optical waveguide 19 from
the input port In3 and enters the circuit and the Y-signal (the
Y-polarization signal split in the PBS 30), which is the Y-polarization
QPSK signal that passes through the optical waveguide 9 and enters the
circuit, are mixed. That is, the 90-degree hybrid circuit 42 mixes and
outputs the Y-polarization signal split in the PBS 30 and local
oscillation light. Due to this, the signal light of the I-channel
component and the signal light of the Q-channel, which is the Y-signal
converted into an intensity-modulated signal, are extracted
independently. The signal light of the I-channel component in Y
polarization passes through output optical waveguides 15, 16 and is
output from the output ports Out5, 6. On the other hand, the signal light
of the Q-channel component passes through output optical waveguides 17,
18 and is output from the output ports Out7, 8, respectively. The signal
light of the I-channel component and the Q-channel component in Y
polarization is input to the B-PD, not shown schematically, via an
optical fiber or without passing therethrough, respectively.

[0106] As described above, the demodulator 1 is a coherent transmission
system demodulator in which the 90-degree hybrid circuits 41, 42 mix the
X-signal and the X-polarization LO light, and the Y-signal and the
Y-polarization LO light, respectively.

[0107] Further, in the demodulator 1, from the output ports Out1 to 4, the
I-channel and the Q-channel of the X-signal are output and from the
output ports Out5 to 8, the I-channel and the Q-channel of the Y signal
are output.

[0108] Then, in the demodulator 1, the plurality of the paths of signal
light, that is, the path of the X-signal light and the path of the
Y-signal light are set so that all the effective optical path lengths
from the input end 4 to the output end 5 are the same.

[0109] For example, in the demodulator 1, the path of the X-signal and the
path of the Y-signal are set so that the difference in the effective
optical path length from the input end 4 to the output end 5 is equal to
or less than a desired value. Here, as an example, it is preferable for a
desired value when the optical path length difference is converted into a
difference in arrival time to be, as an example, 5 ps or less.

[0110] Specifically, settings are done so that the effective optical path
lengths of the four paths from the output part of the PBS 30 to the
output ports Out1 to 4 in the path of the X-signal split by the PBS 30
and the effective optical path lengths of the four paths from the output
part of the PBS 30 to the output ports Out5 to 8 in the path of the
Y-signal split by the PBS 30 are the same, respectively.

[0111] According to the first embodiment having the above configuration,
the following technical advantages can be obtained.

[0112] (1) Within the PLC of the PLC chip 3, the PBS that splits the
DP-QPSK signal 2 into the X-polarization QPSK signal (X-signal) and the
Y-polarization QPSK signal (Y-signal) and the two 90-degree hybrid
circuits 41, 42 for X polarization and Y polarization are integrated.
Because of this, when constituting the DP-QPSK demodulator using the PBS
and the two 90-degree hybrid circuits, alignment work and bonding work
for the optical connection of the PBS and the two 90-degree hybrid
circuits are no longer necessary. As a result of that, it is possible to
eliminate the connection loss between the PBS and the two 90-degree
hybrid circuits and to reduce the manufacturing cost.

[0113] (2) The inventors of the present invention have found such a
problem that the skew between X- and Y-polarization components increases
if the PBS and the 90-degree hybrid circuit are formed on one PLC and
invented a configuration to solve the problem.

[0114] If there is a difference in the path length between the optical
waveguides through which the X-polarization QPSK signal (X-signal) and
the Y-polarization QPSK signal (Y-signal) split in the PBS propagate,
respectively, there occurs a difference between times at which these
signals are output.

[0115] Because of the above, the configuration is set so that the
effective optical path lengths of the four paths from the output part of
the PBS 30 to the output ports Out1 to 4 in the path of the X-signal and
the effective optical path lengths of the four paths from the output part
of the PBS 30 to the output ports Out5 to 8 in the path of the Y-signal
are the same, respectively. Due to this, it is possible to realize a
high-performance PLC-type DP-QPSK demodulator that reduces the skew
between the X- and Y-polarization components.

[0116] For example, it is made possible to reduce the skew between the X-
and Y-polarization components to 5 ps or less in a DP-QPSK modulation
system PLC-type DP-QPSK demodulator with a symbol rate of 25 GSymbol/s
and a bit rate of 100 Gbit/s.

[0117] (PLC-Type DP-QPSK Demodulator According to Second Embodiment)

[0118] FIG. 6 shows a basic configuration of a PLC-type DP-QPSK
demodulator 1A according to a second embodiment.

[0119] In the PLC-type DP-QPSK modulator 1A, the number of the input ports
of LO light is set to one in the PLC-type DP-QPSK demodulator 1 according
to the first embodiment. In the present embodiment, as an example, only
the input port In2 to which the X-polarization LO light is input is
provided as an input port of LO light.

[0120] The demodulator 1A is configured so that the X-polarization LO
light is branched into two within the PLC and each of the branched
X-polarization LO light enters the 90-degree hybrid circuits 41, 42,
respectively. The X-polarization LO light passes through the optical
waveguide 10 and after branched into two to optical waveguides 10a, 10b,
is incident on the 90-degree hybrid circuits 41, 42, respectively.

[0121] In the demodulator 1A, a half-wavelength plate (λ/2 plate) 40
is inserted into the path through which one of each polarization
(X-polarization and Y-polarization) signal (each is modulated according
to independent information and hereinafter, referred to as the X-signal
and the Y-signal) propagates, which is the DP-QPSK signal 2
polarization-split in the PBS 30. The major axis of the wavelength plate
40 makes an angle of 45 degrees with an axis vertical to the direction in
which light is guided and in parallel with the plane of the PLC and the
X-polarization component of the light having passed therethrough is
converted into Y polarization and the Y-polarization component into X
polarization, respectively. Because of this, the X-signal and the
Y-signal are caused to enter each of the 90-degree hybrid circuits 41, 42
in the same polarization state. In the present embodiment, as an example,
the half-wavelength plate 40 is inserted into the optical waveguide 9
through which the Y-signal polarization-split in the PBS 30 propagates,
and therefore, both the X-signal and the Y-signal are caused to enter
each of the 90-degree hybrid circuits 41, 42 in the X-polarization state,
respectively. Because there exists a difference in the effective
refractive index caused by birefringence between each of the polarization
signals polarization-split in the PBS 30, this forms a factor of skew,
however, by adopting the above-mentioned configuration, it is possible to
reduce the skew.

[0122] Other configurations in the demodulator 1A are the same as those in
the demodulator 1 according to the first embodiment.

[0123] According to the second embodiment having the above configuration,
the following technical advantages are obtained in addition to the
technical advantages achieved in the first embodiment.

[0124] The demodulator 1 according to the first embodiment requires two
light sources, that is, the LO light source that outputs X-polarization
LO light and the LO light source that outputs Y-polarization LO light. In
contrast to this, only the LO light source that outputs X-polarization LO
light may be provided as the LO light source in the demodulator 1A
according to the present embodiment, and therefore, it is possible to
further reduce the manufacturing cost of the optical transmission system
configured by using the demodulator 1A.

[0125] In the demodulator 1A, the half-wavelength plate 40 is inserted
into the optical waveguide 9 through which the Y-signal
polarization-split in the PBS 30 propagates, however, it may also be
possible to insert the half-wavelength plate 40 into the optical
waveguide 8 through which the X-signal polarization-split in the PBS 30
propagates. In this configuration, both the X-signal and the Y-signal
enter each of the 90-degree hybrid circuits 41, 42 in the Y-polarization
state, respectively. As described above, by providing the half-wavelength
plate 40 between the PBS 30 and the 90-degree hybrid 41 for the X-signal
or between the PBS 30 and the 90-degree hybrid 42 for the Y-signal, it is
possible to integrate the polarization state of the QPSK signal split by
the PBS 30 into X polarization or Y polarization. Further, it is also
possible to cause the two QPSK signals after polarization is integrated,
that is, the QPSK signal caused to enter the 90-degree hybrid 41 and the
QPSK signal caused to enter the 90-degree hybrid 42 to propagate through
the paths of the same effective refractive index.

[0126] As described above, in the present embodiment, it is possible to
demodulate even the DP-QPSK signal formed by multiplexing two QPSK
signals, that is, the X-polarization QPSK signal and the Y-polarization
QPSK signal, with one of the polarization LO lights and to reduce the
skew resulting from birefringence by providing the half-wavelength plate
40.

[0127] (PLC-Type DP-QPSK Demodulator According to Third Embodiment)

[0128]FIG. 7 shows a basic configuration of a PLC-type DP-QPSK
demodulator 1B according to a third embodiment.

[0129] In the PLC-type DP-QPSK demodulator 1B, three PBSs, that is, the
PBS 30 and PBSs 36, 37, are provided within PLC of the PLC chip 3 in the
PLC-type DP-QPSK demodulator 1 according to the first embodiment. The
second PBS(X) 36 and the second PBS(Y) 37 are cascade-connected to the
first PBS 30, respectively. Further, in the demodulator 1B, in order to
make an attempt to downsize the PLC chip 3, the PBSs 36 and 37 and the
two 90-degree hybrid circuits 41, 42 are put in the proximity of each
other in the spatial arrangement.

[0130] The PBSs 36, 37 are each a MZI that has two couplers and the two
arm waveguides 33, 34 connected between both the couplers as the PBS 30
in the demodulator 1 according to the first embodiment (see FIG. 2).

[0131] In the demodulator 1B, first, the DP-QPSK signal 2 input from the
input port In1 passes through the input optical waveguide 7 and enters
the first PBS 30 and is polarization-split into the X-polarization QPSK
signal (X-signal) and the Y-polarization QPSK signal (Y-signal) by the
first PBS 30. The X-signal and the Y-signal pass through optical
waveguides 38, 39 and enter the second PBS(X) 36 and the second PBS(Y)
37, respectively.

[0132] The second PBS(X) 36 cuts the Y-polarization component included in
the X-signal output from the first PBS 30. Due to this, from the second
PBS(X) 36, the X-signal with a high extinction ratio is output to the
90-degree hybrid circuit (X) 41 for X polarization via the optical
waveguide 8. On the other hand, the second PBS(Y) cuts the X-polarization
component included in the Y-signal output from the first PBS 30. Due to
this, from the second PBS(Y) 37, the Y-signal with a high extinction
ratio is output to the 90-degree hybrid circuit (Y) 42 for Y polarization
via the optical waveguide 9.

[0133] In FIG. 7, the X-polarization LO light and the Y-polarization LO
light are caused to enter the 90-degree hybrid circuit (X) 41 and the
90-degree hybrid circuit (Y) 42, respectively, as in the demodulator 1
according to the first embodiment. Other configurations of the
demodulator 1B are the same as those of the demodulator 1.

[0134] According to the third embodiment having the above configuration,
the following technical advantage is obtained in addition to the
technical advantages achieved in the first embodiment.

[0135] For the X-signal, it is possible to increase the polarization
extinction ratio when entering the 90-degree hybrid (X) 41 by forming a
two-stage configuration of the first PBS 30 and the second PBS(X) 36. For
the Y-signal, it is possible to increase the polarization extinction
ratio when entering the 90-degree hybrid (Y) 42 by forming a two-stage
configuration of the first PBS 30 and the second PBS(Y) 37. Further, the
PBSs 36, 37 and the two 90-degree hybrid circuits 41, 42 are put in the
proximity of each other, respectively, in the spatial arrangement, and
therefore, it is possible to make an attempt to downsize the PLC chip 3
and to realize a compact PLC-type DP-QPSK demodulator.

[0136] (PLC-Type DP-QPSK Demodulator According to Fourth Embodiment)

[0137]FIG. 8 shows a basic configuration of a PLC-type DP-QPSK
demodulator 1C according to a fourth embodiment. In the PLC-type DP-QPSK
demodulator 1C, the three PBSs 30, 36, 37 are provided within the PLC of
the PLC chip 3. Further, in the demodulator 1C, the first PBS 30 in the
first stage, the second PBS(X) 36 and the third PBS(Y) 37 in the second
stage formed in parallel with the first PBS 30 sandwiched in between, and
the 90-degree hybrid circuits 41, 42 formed in parallel with the second
and third PBSs 36, 37 sandwiched in between are provided.

[0138] The first PBS 30 and the second and third PBSs 36, 37 are connected
via folded waveguides 43, 44, respectively, and the second and third PBSs
36, 37 and the 90-degree hybrid circuits 41, 42 are connected via folded
waveguides 45, 46, respectively.

[0139] Here, each folded waveguide includes a bend waveguide having a
fixed curvature radius and a rotation angle of 180 degrees on the PLC
substrate surface.

[0140] In the demodulator 1C, a configuration in which all circuits are
arranged in a narrow region of the PLC chip 3 in the shape of a rectangle
substantially a square is adopted in order to make an attempt to downsize
the PCL chip 3. Other configurations of the demodulator 1C are the same
as those of the demodulator 1.

[0141] According to the fourth embodiment having the above configuration,
it is possible to further downsize the PLC-type DP-QPSK demodulator 1B
according to the third embodiment.

[0142] (PLC-Type DP-QPSK Demodulator According to Fifth Embodiment)

[0143] Next, a PLC-type DP-QPSK demodulator 1D according to a fifth
embodiment is explained based on FIG. 9 to FIG. 18. FIG. 9 shows a basic
configuration of the PLC-type DP-QPSK demodulator 1D according to the
fifth embodiment. In FIG. 11 to FIG. 18, +θ indicates that the
rotation angle θ of the bend waveguide is a plus value and -θ
indicates that the rotation angle θ of the bend waveguide is a
minus value, respectively.

[0144] The PLC-type DP-QPSK demodulator 1D has the following
configurations.

[0145] (1) As shown in FIG. 9, in the demodulator 1D, the long PBS 30 in
the first stage is formed at the central part of the PLC chip 3 in the
shape of a rectangle substantially a square and the long PBSs 36, 37 in
the second stage are formed in parallel with the PBS 30 sandwiched in
between. Further, the 90-degree hybrid circuit 41 is arranged at the
upper side of the PBS in the second stage (the second polarization beam
splitter) 36 in FIG. 9 and the 90-degree hybrid circuit 42 at the lower
side of the PBS in the second stage (the 3rd polarization beam splitter)
37 in FIG. 9, respectively.

[0146] (2) In order to make an attempt to downsize the PLC chip 3, a
configuration is adopted, in which all the circuits of the PBSs 30, 36,
37 and the 90-degree hybrid circuits 41, 42 are arranged in a narrow
region of the rectangular PLC chip 3.

[0147] (3) As shown in FIG. 9 and FIG. 10, the X-polarization LO light
incident on the input port In2 is branched into two parts by a Y-branch
coupler 75 and caused to enter the coupler 21 on the input side of the
90-degree hybrid circuit 41 and the coupler 21 on the input side of the
90-degree hybrid circuit 42 through optical waveguides 76, 77,
respectively. Here, the optical waveguides 76, 77 are formed so as to
cause the X-polarization LO light branched into two by the Y-branch
coupler 75 to enter the couplers 21, 21, respectively, on the input sides
of the 90-degree hybrid circuits 41, by causing it to propagate in the
rightward direction in FIG. 9 after causing it to propagate in the
vertical direction in the diagram from the vicinity of the Y-branch
coupler 75.

[0148] (4) As shown in FIG. 9 and FIG. 10, the DP-QPSK signal 2 incident
on the input port In1 passes through an optical waveguide 73 that
bypasses the Y-branch coupler 75 and is input to the coupler 31 on the
input side of the PBS 30.

[0149] (5) As shown in FIG. 9 and FIG. 11, the coupler 32 on the output
side of the PBS 30 in the first stage and a coupler 36a on the input side
of the PBS 36 in the second stage formed at the upper side thereof are
connected by the folded waveguide 43 including bend waveguides, which
have the absolute value of the total of the rotation angles the sign of
which is not reversed being greater than 180 degrees, that is, the total
of the rotation angles θ being greater than +180 degrees, from the
output end of the optical function part (the coupler 32) on the side of
the previous stage toward the input end of the optical function part (the
coupler 36a) on the side of the post stage.

[0150] In the present specification, in the rectangular PLC chip 3, a
direction in which light is incident on the input port is referred to as
an x-direction (transverse direction in FIG. 9) and a direction vertical
to the x-direction is referred to as a y-direction.

[0151] By setting the rotation angle θ of the folded waveguide 43
that combines a bend waveguide of +θ and a bend waveguide of
-θ so that the absolute value of the total of the rotation angle
the sign of which is not reversed from the output end of the optical
function part on the side of the previous stage toward the input end of
the optical function part on the side of the post stage to the following
range, it is possible to suppress an increment of the PLC chip 3 in the
y-direction. That is, it is possible to reduce the size of the PLC chip 3
in the vertical direction, that is, the size in direction in which the
optical function parts are arranged side by side when each optical
function part is integrated in parallel on the PLC.

180°≦θ≦270°

[0152] If the rotation angle θ is less than 180 degree, the amount
of advancement of the folded waveguide 43 in the negative x-direction (in
the leftward direction in FIG. 9) from the side of the output end 5
toward the side of the input end is small, and this is unfavorable. If
the rotation angle θ exceeds 270°, the end point of the
folded waveguide 43 does not advance in the upward direction in the
diagram, that is, it advances to the opposite side of the optical
function part on the side of the post stage arranged in parallel (the
negative y-direction from the start point), and this is unfavorable.

[0153] The folded waveguide 43 includes a bend waveguide 43a of
+90°, a bend waveguide 43b of +90°, and a bend waveguide
43c of several degrees in order from the side of the coupler 32 on the
output side.

[0154] As shown in FIG. 12, the bend waveguide 43c has a bend waveguide
43c1 of plus several degrees and a bend waveguide 43c2 of minus several
degrees and the bend waveguide 43c2 is connected to the coupler 36a on
the input side of the PBS 36 via a straight waveguide 43c3.

[0155] Due to this, it is possible to arrange the PBS 36 in the second
stage in parallel and in the proximity so as to sandwich the PBS 30 in
the first stage on the PLC chip 3 while securing a bend radius in such a
degree that the loss due to leaked light is not problematic for each of
the bend waveguides 43a, 43b, 43c.

[0156] (6) As shown in FIG. 9 and FIG. 13, the coupler 32 on the output
side of the PBS 30 in the first stage and a coupler 37a on the input side
of the PBS 37 in the second stage formed in parallel at the lower part
thereof in the diagram are connected by the folded waveguide 44 including
bend waveguides, which have the absolute value of the total of rotation
angles the sign of which is not reversed, that is, the total of the
rotation angles θ being less than -180 degrees, from the output end
of the optical function part (the coupler 32) on the side of the previous
stage toward the input end of the optical function part (the coupler 37a)
on the side of the post stage. In this case, the bends having a minus
rotation angle (-θ) continue and the absolute value of the total of
the rotation angles is greater than 180 degrees.

[0157] It is preferable to set the rotation angle θ of the folded
waveguide 44 that combines a bend waveguide of -θ and a bend
waveguide of +θ to the same range as that of the folded waveguide
43. The folded waveguide 44 includes a bend waveguide 44a of -90°,
a bend waveguide 44b of -90°, and a bend waveguide 44c of several
degrees in order from the side of the coupler 32 on the output side.

[0158] As shown in FIG. 14, the bend waveguide 44c has a bend waveguide
44c1 of minus several degrees and a bend waveguide 44c2 of plus several
degrees and the bend waveguide 44c2 is connected to the coupler 37a on
the input side of the PBS 37 via a straight waveguide 44c3.

[0159] Due to this, it is possible to arrange the PBS 37 in the second
stage in parallel and in the proximity so as to sandwich the PBS 30 in
the first stage on the PLC chip 3 while securing a bend radius in such a
degree that the loss due to leaked light is not problematic for each of
the bend waveguides 44a, 44b, 44c.

[0160] (7) As shown in FIG. 9 and FIG. 15, a coupler 36b on the output
side of the PBS 36 in the second stage and the coupler 20 on the input
side of the 90-degree hybrid circuit 41 formed at the upper part thereof
(in the positive y-direction (in the upward direction in FIG. 9)) are
connected by the folded waveguide 45 that combines bend waveguides of the
plus rotation angle (+θ) and bend waveguides of the minus rotation
angle (-θ), which have the absolute value of the total of rotation
angles the sign of which is not reversed being greater than 180 degrees,
from the output end of the optical function part (the coupler 36b) on the
side of the previous stage toward the input end of the optical function
part (the coupler 20) on the side of the post stage.

[0161] It is also preferable to set the rotation angle θ of the
folded waveguide 45 that combines the bend waveguides of +θ and the
bend waveguides of -θ to the same range as that of the bend
waveguide 43. The folded waveguide 45 has a bend waveguide 45a of plus
tens of degrees, a bend waveguide 45b of minus tens of degrees, a bend
waveguide 45c of -90°, a straight waveguide 45d, a bend waveguide
45e of -90°, a bend waveguide 45f of about -45°, and a bend
waveguide 45g of about +45° in order from the side of the coupler
36b on the output side. Due to this, it is possible to arrange the PBS 36
in the second stage and the 90-degree hybrid circuit 41 in parallel with
and in the proximity of each other on the PLC chip 3 while securing a
bend radius in such a degree that the loss due to leaked light is not
problematic for each of the bend waveguides 45a, 45b, 45c, 45e, 45f, and
45g. To complement this, the folded waveguide 45 includes the straight
waveguide 45d in the middle thereof, however, the rotation angle of the
straight waveguide 45d is zero, and therefore, the sign of the angle
θ is not reversed.

[0162] Further, the configuration is such that in which output light from
the PBS 36 configured as the Mach-Zehnder interferometer passes through a
cross port of the coupler 36b on the output side when viewed from the
folded waveguide 43 and enters the folded waveguide 45. Due to this, it
is possible to increase the extinction ratio of PBS and at the same time,
to increase the bend radius of the folded waveguide 45.

[0163] (8) As shown in FIG. 9 and FIG. 16, a coupler 37b on the output
side of the PBS 37 in the second stage and the coupler 20 on the input
side of the 90-degree hybrid circuit 42 formed at the lower part thereof
(in the negative y-direction (in the downward direction in FIG. 9)) are
connected by the folded waveguide 46 that combines bend waveguides of the
plus rotation angle (+θ) and bend waveguides of the minus rotation
angle (-θ), which have the total of rotation angles (the plus
rotation angles θ) in the direction from the output end of the
optical function part (the coupler 37b on the output side) on the side of
the previous stage toward the input end of the optical function part (the
coupler 20 on the input side) on the side of the post stage being greater
than 180 degrees.

[0164] It is also preferable to set the rotation angle θ of the
folded waveguide 46 that combines the bend waveguides of +θ and the
bend waveguides of -θ to the same range as that of the bend
waveguide 43. The folded waveguide 46 has a bend waveguide 46a of minus
several degrees, a bend waveguide 46b of plus several degrees, a bend
waveguide 46c of +90°, a straight waveguide 46d, a bend waveguide
46e of +90°, a bend waveguide 46f of about +45°, and a bend
waveguide 46g of about -45° in order from the side of the coupler
37b on the output side. Due to this, it is possible to arrange the PBS 37
in the second stage and the 90-degree hybrid circuit 42 in parallel with
and in the proximity of each other on the PLC chip 3 while securing a
bend radius in such a degree that the loss due to leaked light is not
problematic for each of the bend waveguides 46a, 46b, 46c, 46e, 46f, and
46g. To complement this, the folded waveguide includes the straight
waveguide 46d in the middle thereof, however, the rotation angle of the
straight waveguide 46d is zero, and therefore, the sign of the angle
θ is not reversed.

[0165] Further, the configuration is such that in which output light from
the PBS 37 configured as the Mach-Zehnder interferometer passes through a
cross port of the coupler 37b on the output side when viewed from the
folded waveguide 44 and enters the folded waveguide 46. Due to this, it
is possible to increase the extinction ratio of PBS and at the same time,
to increase the bend radius of the folded waveguide 46.

[0166] (9) The half-wavelength plate (λ/2 plate) 40 is inserted into
the path through which one of signal light (each is modulated by
independent information and hereinafter, referred to as the X-signal and
the Y-signal) of each polarization component (X polarization and Y
polarization) of the DP-QPSK signal 2 polarization-split in the PBS 30
propagates. Due to this, the X-signal and the Y-signal are caused to
enter the 90-degree hybrid circuits 41, 42, respectively, in the same
polarization state. In the present embodiment, as an example, the
half-wavelength plate 40 is inserted into the straight waveguide 46d of
the folded waveguide 46 through which the Y-signal polarization-split in
the PBS 30 passes, and therefore, both the X-signal and the Y-signal are
caused to enter the 90-degree hybrid circuits 41, 42, respectively, in
the X-polarization state.

[0167] (10) For each of the folded waveguides 43, 44 and folded waveguide
47, 48, in order to set the radius of the bend waveguide to an optimum
value, for example, 2,000 μm to 1,800 μm, the width of each bend
waveguide is increased to, for example, 7 μm, respectively. At the end
point of each folded waveguide, the width of the waveguide is converted
into 6 μm again by a taper and the bend radius of the rest of the
waveguide is set to 2,000 μm.

[0168] The reason for the above is to meet the demand that the separation
between the output ports Out1, 2 and the output ports Out3, 4, the
separation between the output ports Out3, 4 and the output ports Out5, 6,
and the separation between the output ports Out 5, 6 and the output ports
Out 7, 8 be the same (for example, a separation of 6 mm) within the
limited size of the PLC chip 3.

[0169] (11) The configuration is set so that it is possible to cause light
to enter inspection input ports P1, P4 of the PBSs 36, 37 in the second
stage and to cause light to pass through only the PBSs 36, 37 and exit
from inspection output ports P2, P3 for the inspection/adjustment of the
PBSs 36, 37.

[0170] Specifically, as shown in FIG. 9, the inspection input port P1 is
connected to one of the input ports of the coupler 36a on the input side.
Further, as shown in FIG. 9 and FIG. 17, the through port of the coupler
36b on the output side of the PBS 36 and an optical waveguide 71
connected to the inspection output port P2 are connected by the folded
waveguide 47 that combines bend waveguides of the plus rotation angle
(+θ) and bend waveguides of the minus rotation angle (-θ),
which have the total of the rotation angles (plus rotation angles
θ) the sign of which is not reversed being greater than +180
degrees, from the output end of the optical function part (the coupler
36b) on the side of the previous stage toward the optical waveguide 71.

[0171] As shown in FIG. 17, the folded waveguide 47 has a bend waveguide
47a of plus tens of degrees, a bend waveguide 47b of +90°, a bend
waveguide 47c of +90°, a bend waveguide 47d of about -45°,
and a bend waveguide 47e of about +45° in order from the side of
the coupler 36b. Due to this, it is possible to connect the PBS 36 and
the optical waveguide 71 in parallel with each other in close positions
while securing a bend radius in such a degree that the loss due to leaked
light is not problematic for each of the bend waveguides 47a, 47b, 47c,
47d, and 47e.

[0172] On the other hand, the inspection input port P4 is connected to one
of the input ports of the coupler 37a on the input side of the PBS 37.
Further, as shown in FIG. 9 and FIG. 18, the through port of the coupler
37b on the output side of the PBS 37 and an optical waveguide 72
connected to the inspection output port P3 are connected by the folded
waveguide 48 that combines bend waveguides of the plus rotation angle
(+θ) and bend waveguides of the minus rotation angle (-θ),
which have the absolute value of the total of the rotation angles θ
the sign of which is not reversed being greater than 180 degrees, from
the output end of the optical function part (the coupler 37b) on the side
of the previous stage toward the optical waveguide 72. The optical
waveguide 72 is bent and extends as the optical waveguide 71 in a
position close to the optical waveguide 71.

[0173] As shown in FIG. 18, the folded waveguide 48 has a bend waveguide
48a of several degrees, a bend waveguide 48b of -90°, a bend
waveguide 48c of -90°, a bend waveguide 48d of about +45°,
and a bend waveguide 48e of about -45° in order from the side of
the coupler 37b on the output side. Due to this, it is possible to
connect the PBS 37 and the optical waveguide 72 in parallel with each
other in close positions while securing a bend radius in such a degree
that the loss due to leaked light is not problematic for each of the bend
waveguides 48a, 48b, 48c, 48d, and 48e.

[0174] In each of the Mach-Zehnder interferometers constituting the PBSs
30, 36, 37, a heater is disposed in at least one of the upper and lower
arms. By adjusting the voltage to be applied to the heater and
controlling the refractive index and the amount of birefringence of the
waveguide independently, it is made possible to make an adjustment so
that the polarization extinction ratio is a desired value as the PBS. At
that time, it is possible to adjust only the PBS 36 by causing light to
enter from P1 and measuring the light output from P2. Further, it is
possible to adjust only the PBS 37 by causing light to enter from P4 and
measuring the light output from P3. It may be also possible to exchange
input/output of P1 and P2 and also to exchange input/output of P3 and P4.
After the adjustment of the PBS 36 is completed, by causing light to
enter from P2 and measuring the light output from In1 or In2, it is
possible to adjust the PBS 30. Alternatively, after the adjustment of the
PBS 37 is completed, by causing light to enter from P3 and measuring the
light output from In1 or In2, it is also possible to adjust the PBS 30.

[0175] Other configurations in the demodulator 1D are the same as those in
the demodulator 1 according to the first embodiment.

[0176] According to the fifth embodiment having the above configuration,
the following technical advantages are obtained in addition to the
technical advantages achieved in the first embodiment.

[0177] By the above configurations (1), (2), it is possible to make an
attempt to downsize the PLC chip 3 and to realize a compact PLC-type
DP-QPSK demodulator.

[0178] By the above configurations (5) to (8), it is possible to arrange
the PBSs 36, 37 and the two 90-degree hybrid circuits 41, 42 in parallel
with and in the proximity of each other in the PLC chip 3 while securing
a bend radius in such a degree that the loss due to leaked light is not
problematic for each bend waveguide. Consequently, it is possible to
realize a compact PLC-type DP-QPSK demodulator without deteriorating the
optical characteristic. By the above configuration (11), it is possible
to independently adjust a plurality of PBSs integrated on one PLC.

[0179] By arranging the PBS 30 and the long PBSs 36, 37 in close positions
on the PLC chip 3, it is possible to realize a high-performance PLC-type
DP-QPSK demodulator that reduces the skew between the X-signal and the
Y-signal.

[0180] Further, the characteristics of the waveguides (for example, the
operation as an interferometer) become close to one another, and
therefore, the labor and time required for adjustment can be omitted.
That is, when a plurality of Mach-Zehnder interferometers exits on the
PLC chip 3, if the values of their refractive indexes are the same, the
interferometers having the same arm-to-arm optical path difference
(physical length) exhibit the same interference conditions (the value of
FSR, the wavelength that gives the peak value of the transmission
function, etc.), and therefore, labor and time required for adjustment
can be omitted.

[0181] Further, the configuration is such that light passes through the
cross ports, respectively, of the couplers 36b, 37b on the output side of
the PBSs 36, 37 when viewed from the input side waveguides 43 and 44 and
enters the folded waveguides 45, 46. Due to this, it is possible to
increase the extinction ratio of PBS and at the same time, to increase
the bend radius of the folded waveguides 45, 46, respectively.

[0182] As described above, according to the present embodiment, the
polarization beam splitter, the 90-degree hybrid circuit, and the
polarization beam splitters in the first stage and the second stage
formed respectively in parallel on the rectangular PLC chip are connected
by bend waveguides as a folded waveguide, which have the absolute value
of the total of the rotation angles the sign of which is not reversed
being greater than 180 degrees, from the output end of the optical
function part on the side of the previous stage toward the input end of
the optical function part on the side of the post stage. Due to this, it
is possible to arrange a plurality of polarization beam splitters and two
90-degree hybrid circuits in parallel with and in the proximity of one
another while securing a bend radius in such a degree that the loss due
to leaked light is not problematic for each bend waveguide. Consequently,
it is possible to realize a compact PLC-type DP-QPSK demodulator without
deteriorating the optical characteristics.

[0183] (PLC-Type DP-QPSK Demodulator According to Sixth Embodiment)

[0184]FIG. 19 shows a basic configuration of a PLC-type DP-QPSK
demodulator 1E according to a sixth embodiment. The main different point
between the demodulator 1E and the demodulator 1D shown in FIG. 9 lies in
the following configuration.

[0185] (1) A bent taper is used to turn parts of the folded waveguides 45,
46 into the wide waveguides 45a, 46a for a wavelength plate slit,
respectively. The bent taper is a portion where each of the wide
waveguides 45a, 46a bends upward/downward in the diagram. The "bent
taper" here means a waveguide that converts the width while bending, that
is, a waveguide the width of which varies.

[0186] (2) In order to reduce the longitudinal size of the PLC chip 3, at
the output end 5, the eight output ports Out1 to 4 and Out5 to 8 are
respectively arranged integrally.

[0187] (3) Light incident from the inspection port P1 passes through the
PBS 36 and then passes through the folded waveguide 47 and the optical
waveguide 71, and then, is output from the inspection port P2. On the
other hand, light incident from the inspection port P4 passes through the
PBS 37 and then passes through the folded waveguide 48 and the optical
waveguide 72, and then, is output from the inspection port P3. In this
manner, in the present embodiment, one of the optical waveguides 71, 72
is formed at the upper side of the PBS 30 (in the positive y-direction
(in the upward direction in FIG. 19)) and the other is formed at the
lower side of the PBS 30 (in the negative y-direction (in the downward
direction in FIG. 19)).

[0188] As described above, the present invention is characterized by using
bend waveguides, which have the absolute value of the total of the
rotation angles the sign of which is not reversed being greater than 180
degrees, from the output end of the optical function part (coupler) on
the side of the previous stage toward the optical function part (coupler)
on the side of the post stage when integrating a plurality of optical
function parts in parallel on one PLC. Due to this, the configuration is
such that after the bend waveguides are caused to bulge out excessively
to the side of the optical function parts on the side of the previous
stage and/or the side of the post stage and then part thereof is
returned, and therefore, the degree of freedom of the separation between
the optical function parts integrated in parallel on the side of the
previous stage and the side of the post stage and the arrangement of each
optical function part in the lengthwise direction is improved and it is
possible to realize downsizing of an optical integrated circuit.

[0189] Further, by making the absolute value of the total of the rotation
angles by which the bend waveguides are bent successively in one
direction greater than 180 degrees, it is possible to realize downsizing
of an optical integrated circuit more effectively.

[0190] It is desirable for a bend waveguide used in the present invention
to have a fixed curvature radius in such a degree that the excessive loss
due to bending is not problematic.

[0191] However, the wording "the sign is not reversed", "bending in one
direction", etc., in the present invention do not exclude a small
difference that does not affect downsizing of an optical integrated
circuit and should be accepted appropriately based on the gist of the
present invention.

[0192] (PLC-Type DP-QPSK Demodulator According to Seventh Embodiment)

[0193] FIG. 21 shows a basic configuration of a PLC-type DP-QPSK
demodulator according to a seventh embodiment.

[0194] In the demodulator 1E according to the present embodiment, the path
through which an input QPSK signal and LO light enter the 90-degree
hybrid circuit is folded twice and between a region folded for the first
time (region where the input light (QPSK signal and LO light) is changed
to the opposite side of the input direction) and a region folded for the
second time (region where the light traveling on the opposite side is
changed again to the input direction), PBSs for the QPSK signal and LO
light are provided, respectively. Then, the structure of the second time
folded region is devised so that the optical path length for the X-signal
(X-polarization QPSK signal) from the PBS to the 90-degree hybrid circuit
and the optical path length for the Y-signal (Y-polarization QPSK signal)
from the PBS to the 90-degree hybrid circuit are at least the same.

[0195] In FIG. 21, at the input end 4 of the demodulator 1E, the input
port In1 of the DP-QPSK signal as a polarization-multiplexed coherent
modulated signal and an input port In4 to input both the X-polarization
LO light and the Y-polarization LO light are provided. Further, at a
predetermined distance apart from the input end 4, the PBS 30 to split
the DP-QPSK signal into the X-signal and the Y-signal and a PBS 30a to
split the X-polarization and Y-polarization multiplexed LO light (light
that mixes the X-polarization LO light and the Y-polarization LO light)
into the X-polarization LO light and the Y-polarization LO light are
provided. That is, the PBS 30 and the PBS 30a are arranged in parallel at
the same distance apart from the input end 4, respectively. The PBS 30a
has the same structure as that of the PBS 30 shown in FIG. 2 and is a
Mach-Zehnder interferometer (MZI) having the two couplers 31a, 32a and
two arm waveguides connected between both the couplers 31a, 32a.

[0196] In the post stage of the input port In1, a waveguide 101 having a
first folded region 101a and connecting the input port In1 and the
coupler 31 of the PBS 30 (the input side coupler of the PBS 30) is
provided and in the post stage of the input port In4, a waveguide 102
having a first folded region 102a and connecting the input port In4 and
the coupler 31a of the PBS 30a (the input side coupler of the PBS 30a) is
provided. On the side of the post stage in the direction of the
propagation of the QPSK signal (X-signal) of the PBS 30, the 90-degree
hybrid circuit 41 is provided and on the side of the post stage in the
direction of the propagation of the QPSK signal (Y-signal) of the PBS
30a, the 90-degree hybrid circuit 42 is provided.

[0197] The coupler 32 of the PBS 30 (the output side coupler of the PBS
30) and the Y-branch coupler 20 of the 90-degree hybrid circuit 41 are
connected by a waveguide 103 having a second folded region 103a and the
coupler 32 of the PBS 30 and the Y-branch coupler 20 of the 90-degree
hybrid circuit 42 are connected by a waveguide 104 having a second folded
region 104a. Further, the coupler 32a of the PBS 30a (the output side
coupler of the PBS 30a) and the Y-branch coupler 21 of the 90-degree
hybrid circuit 41 are connected by a waveguide 105 having a second folded
region 105a and the coupler 32a of the PBS 30a and the Y-branch coupler
21 of the 90-degree hybrid circuit 42 are connected by a waveguide 106
having a second folded region 106a.

[0198] In FIG. 21, along the direction of an arrow Q perpendicular to the
direction of an arrow P from the input end 4 toward the output end 5 in
opposition to the input end 4, the 90-degree hybrid circuit 41, the PBS
30, the PBS 30a, and the 90-degree hybrid circuit 42 are arranged in this
order. Consequently, the waveguide 103 for the X-signal to propagate
intersects the waveguide 105 for the X-polarization LO light to propagate
and the waveguide 104 for the Y-signal to propagate intersects the
waveguide 105 and the waveguide for the Y-polarization LO light to
propagate.

[0199] The main different point between the demodulator 1E and the
demodulator 1D shown in FIG. 9 lies in the following configuration.
First, in the modulator 1E, the PBS 30 for signal light is configured by
a MZI in one stage. Further, the PBS 30a for LO light is also configured
by a MZI in one stage and integrated on the PLC chip 3. The arrangement
of the input waveguide and the folded waveguide of the signal light and
LO light, and the PBS has a configuration axisymmetric with respect to
the direction of the arrow P (transverse direction) from the input end 4
toward the output end 5 in opposition to the input end 4. Further, by
configuring the four waveguides 103 to 106 after exiting the PBSs 30, 30a
into the configuration (details will be described later) in FIG. 22, the
four waveguides through which the X-polarization component (X-signal) and
the Y-polarization component (Y-signal) of the signal light (QPSK signal)
and the X-polarization component and the Y-polarization component of the
LO light propagate are folded in a symmetric form. Consequently, it is
possible to reduce the occurrence of skew and to guide the polarization
components to the two 90-degree hybrid input ports corresponding to the
polarization components, respectively. This configuration enables a
folded configuration in which the occurrence of skew is reduced and
further contributes greatly to downsizing in chip size (in particular, in
the transverse direction).

[0200] In the present embodiment, in order to reduce the occurrence of
skew as described above, the optical path length of the path (the
waveguide 103) for X-signal between the PBS 30 and the 90-degree hybrid
circuit 41 and that of the path (the waveguide 104) for Y-signal between
the PBS 30 and the 90-degree hybrid circuit 42 are made the same.
Further, the optical path length of the path (the waveguide 105) for
X-polarization LO light between the PBS 30a and the 90-degree hybrid
circuit 41 and that of the path (the waveguide 106) for Y-polarization LO
light between the PBS 30a and the 90-degree hybrid circuit 42 are also
made the same. In order to realize the above, the present embodiment is
characterized in the structure of the second folded region as shown in
FIG. 22.

[0201] FIG. 22 is a diagram for explaining the configuration of the second
folded region of the waveguides 103 to 106. In FIG. 22, the waveguide 103
has a bend waveguide (bend waveguide of the rotation angle +θ) 103b
the same shape as a fan-shaped arc of the bend radius r and a central
angle θ, a straight waveguide 103c of a predetermined length l, a
bend waveguide (bend waveguide of a rotation angle +(π-2θ)) 103d
the same shape as a fan-shaped arc of the bend radius r and a central
angle (π-2θ)), and a remaining waveguide 103e. The waveguide 104
has a bend waveguide (bend waveguide of a rotation angle -θ) 104b
the same shape as a fan-shaped arc of the bend radius r and the central
angle θ, a straight waveguide 104c of the predetermined length l, a
bend waveguide (bend waveguide of a rotation angle -(π-2θ)) 104d
the same shape as a fan-shaped arc of the bend radius r and the central
angle (π-2θ)), and a remaining waveguide 104e. The waveguide 105
has a bend waveguide 105b of the rotation angle +θ, a straight
waveguide 105c of the predetermined length l, a bend waveguide 105d of
the rotation angle +(π-2θ), and a remaining waveguide 105e.
Further, the waveguide 106 has a bend waveguide 106b of the rotation
angle -θ, a straight waveguide 106c of the predetermined length l,
a bend waveguide 106d of the rotation angle -(π-2θ), and a
remaining waveguide 106e.

[0202] As described above, the shape of the waveguide 103 from the bend
waveguide 103b to the bend waveguide 103d and the shape of the waveguide
105 from the bend waveguide 105b to the bend waveguide 103d are the same
and the shape of the waveguide 104 from the bend waveguide 104b to the
bend waveguide 104d and the shape of the waveguide 106 from the bend
waveguide 106b to the bend waveguide 106d are the same. Further, the
shape of the waveguide 103 from the bend waveguide 103b to the bend
waveguide 103d and the shape of the waveguide 104 from the bend waveguide
104b to the bend waveguide 104d are axisymmetric with respect to the
direction of the arrow P.

[0203] In the present embodiment, the waveguide 104, which is the path
from the PBS 30 toward the 90-degree hybrid circuit 42, and the waveguide
105, which is the path from the PBS 30a toward the 90-degree hybrid
circuit 41, intersect with each other at the boundary between the
straight waveguide 104c and the bend waveguide 104d and the boundary
between the straight waveguide 105 and the bend waveguide 105d and form
an intersection angle 2θ. It is assumed that the distance (DC
pitch) between two waveguides arranged in the proximity of each other in
the couplers 32, 32a is p. At this time, if the line length l is
determined as follows, it is possible to make the same the shapes of the
four waveguides 103, 104, 105, 106 as described above, and to make
2θ all the intersection angles when the waveguide 103 intersects
the waveguide 105, the waveguide 105 intersects the waveguide 104, and
the waveguide 104 intersects the waveguide 106.

[0204] First, the distance along the direction of the arrow Q from the
central line of the PBS 30, that is, the central line (both are lines
parallel with the arrow p in FIG. 21) to the point where the straight
waveguide 104c and the straight waveguide 105c intersect with each other
is p/2+r(1-cos θ)+l sin θ. What is required is to cause this
length to agree with r sin(π/2-θ)=r cos θ, the distance
along the direction of the arrow Q of the first half part of the bend
waveguide 105d of the rotation angle -(π-2θ), that is the bend
waveguide of the rotation angle -(π/2-θ). From this,

p/2+r(1-cos θ)+l sin θ=r cos θ (1)

holds and from the expression (1),

[Mathematical expression 1]

l=(2r cos θ-r-p/2)/sin θ (2)

is obtained.

[0205] As described above, it is also possible to uniquely determine the
length l of the straight waveguides 103c, 104c, 105c, 106c by determining
the bend radius r and the angle θ. That is, the bend waveguides
103b, 104b, 105b, 106b located in the previous stage of the straight
waveguides 103c, 104c, 105c, 106c correspond to the arc on the sector
side in the same shape and the bend waveguides 103d, 104d, 105d, 106d
located in the post stage of the straight waveguides 103c, 104c, 105c,
106c also correspond to the arc on the sector side in the same shape.
Because of this, by finding the length l of each of the straight
waveguides 103c, 104c, 105c, 106c according to the expression (2), it is
possible to make the same the optical path lengths of the straight
waveguides 103c, 104c, 105c, 106c and as a result of that, it is possible
to at least make the same shape or an axisymmetric shape up to the
regions where the waveguide is folded (the waveguide 103; the bend
waveguide 103b, the straight waveguide 103c, the bend waveguide 103d, the
waveguide 104; the bend waveguide 104b, the straight waveguide 104c, the
bend waveguide 104d, the waveguide 105; the bend waveguide 105b, the
straight waveguide 105c, the bend waveguide 105d, the waveguide 106; the
bend waveguide 106b, the straight waveguide 106c, the bend waveguide
106d) of the waveguides 103 to 106. Because of this, it is possible to
make the same the optical path lengths of the regions where the waveguide
is folded of the waveguides 103 to 106.

[0206] As to the remaining waveguides 103e, 104e, 105e, 106e, the paths
after the waveguide is folded, it is not necessary to make their shapes
the same, however, the optical path lengths up to the 90-degree hybrid
circuit (corresponding Y-branch coupler) are set the same.

[0207] Consequently, it is possible to make the same the optical path
lengths of the waveguides 103 to 106 and to reduce the occurrence of
skew.

[0208] An example of a method of designing the waveguides 103 to 106 in
the present embodiment is explained. In accordance with the
specifications of the demodulator, the curvature radii r of the bend
waveguides 103b, 104b, 105b, 106b and the bend waveguides 103d, 104d,
105d, 106d are determined and the DC pitch p of the couplers 32, 32a is
determined. The bend radii of the bend waveguides 103b, 104b, 105b, 106b
and the bend waveguides 103d, 104d, 105d, 106d are the same value. Next,
the intersection angle 2θ of the waveguide 104 and the waveguide
105 is determined. For example, when it is desired to make the loss due
to the intersection of the waveguides 104, 105 a minimum, the
intersection angle 2θ is set to 90 degrees.

[0209] When the intersection angle 2θ is determined in this manner,
the shapes of the bend waveguides 103b, 104b, 105b, 106b (the rotation
angle θ) and the bend waveguides 103d, 104d, 105d, 106d (the
rotation angle (π-2θ)) are determined because the bend radii r
of the respective bend waveguides are determined. By substituting the
bend radius r, the DC pitch p, and the angle θ into the expression
(2), it is possible to determine the lengths 1 of the straight waveguides
103c, 104c, 105c, 106c and the shapes of the straight waveguides 103c,
104c, 105c, 106c are determined.

[0210] As described above, in the present embodiment, the intersection
angle is included in the design parameters of the straight waveguides and
the bend waveguides in the previous stage and the post stage of the
straight waveguides, and therefore, it is possible to easily design the
structure according to the intersection angle, in which the folding of
the waveguide is realized and the optical path lengths of the waveguide
portions that realize the folding are made the same.

[0211] The absolute value of the rotation angles of the bend waveguides
103d, 104d, 105d, 106d is limited to (π-2θ). The reason is that
when the absolute value of the rotation angle exceeds (π-2θ),
the excess portion may be regarded to be included in the remaining
waveguides 103e, 104e, 105e, 106e and conversely, when the absolute value
of the rotation angle is less than (π-2θ), it is not possible to
set the intersection angle at which the bend waveguide 105d and the bend
waveguide 103d intersect with each other (or the bend waveguide 105d and
the straight waveguide 103c intersect with each other) to 2θ and
similarly, it is not possible to set the intersection angle at which the
bend waveguide 104d and the bend waveguide 106d intersect with each other
(or the bend waveguide 104d and the straight waveguide 106c intersect
with each other) to 2θ.

[0212] (PLC-Type DP-QPSK Demodulator According to Eighth Embodiment)

[0213] FIG. 23 shows a basic configuration of a PLC-type DP-QPSK
demodulator according to an eighth embodiment.

[0214] A demodulator 1F shown in FIG. 23 has the same basic structure as
that of the demodulator 1E according to the seventh embodiment shown in
FIGS. 21, 22, however, the specific refractive index difference between
the core and cladding of the waveguide is set to, as an example, 1.2% in
the case of the demodulator 1E and 1.8% in the case of the demodulator 1F
shown in FIG. 23. Because of this, it is possible to reduce the bend
radius of the bend waveguide from 2 mm to 1.2 mm. As a result of that, it
is possible to reduce the transverse length of the demodulator 1F of the
present embodiment to 15 mm and the longitudinal length to 13 mm while
the transverse length (the length in the direction of the arrow P in FIG.
21) of the demodulator 1E is 25 mm and the longitudinal length (the
length in the direction of the arrow Q in FIG. 21) is 16 mm.

[0215] In the present embodiment, a bent part is provided in the PBSs 30,
30a, and therefore, a heater to be formed is also bent. Consequently, as
shown in FIG. 23, on the arm waveguide of the PBS 30, a heater 110 in a
straight shape and a heater 111 in a bent shape are formed and on the arm
waveguide of the PBS 30a, a heater 111a in a straight shape and a heater
110a in a bent shape are formed. By bending the heater, it is possible to
reduce the size in the transverse direction compared to the case where
the heater of equal length is not bent. A demodulator 1G in which the
heaters 110, 110a in the straight shape in the demodulator 1F are omitted
in order to make an attempt to further reduce the size is shown in FIG.
24. With the demodulator 1G shown in FIG. 24, it is possible to further
reduce the chip size by omitting the heaters 110, 110a in the straight
shape and to realize the size of 12 mm for both the longitudinal and the
transverse directions.

[0216] <Optical Transmission System>

[0217] Next, an embodiment of an optical transmission system 50 using the
PLC-type DP-QPSK demodulator explained in each of the embodiments is
explained based on FIG. 20.

[0218] In the optical transmission system 50, as an example, the PLC-type
DP-QPSK demodulator 1A according to the third embodiment shown in FIG. 6
is used. The optical transmission system 50 shown in FIG. 20 comprises a
transmitter 51 that phase-modulates a transmission signal and outputs a
DP-QPSK signal, an optical transmission path 52 configured by an optical
fiber, an erbium-doped fiber amplifier (EDFA) 53, an AWG 54, and a
receiver 55.

[0219] The transmitter 51 outputs the DP-QPSK signal 2 formed by
wave-multiplexing the X-polarization QPSK signal and the Y-polarization
QPSK signal, which are quadrature phase shift keyed X-polarization light
and Y-polarization light of a plurality of wavelengths (λ1 to
λn). That is, from the transmitter 51 to the optical
transmission path 52, a DP-QPSK signal corresponding to n waves of the
multiplexed DP-QPSK signal 2 of a plurality of wavelengths is output.

[0220] The receiver 55 comprises an LO light source 56 that outputs
X-polarization LO light, the PLC-type DP-QPSK demodulator 1A, four
balanced photodiodes (B-PD), that is, the balanced photodiodes 61 and 62
and balanced photodiodes 63 and 64, each having a pair of photodiodes,
and a digital signal processing circuit (DSP) 65.

[0221] The P-PDs 61, 63 are each an optical detector for the I channel and
the B-PDs 62, 64 are each an optical detector for the Q channel. The
digital signal processing circuit 65 comprises a clock extraction circuit
that reproduces a clock having the same rate as and in synchronization
with a demodulated signal obtained by demodulating each of X-polarization
and Y-polarization QPSK signals, a sampling circuit for the I channel and
the Q channel that performs sampling with the clock, an A/D converter
that converts each sampling signal into a digital signal, etc.

[0222] In the optical transmission system 50 having the above
configuration, the DP-QPSK signal corresponding to n waves output from
the transmitter 51 propagates through the optical transmission path 52
and after amplified in the EDFA 53, enters the AWG 54 and separated by
the AWG 54. Among the light of a plurality of wavelengths (λ1
to λn) separated by the AWG 54, for example, the DP-QPSK
signal of a wavelength λi is input to the input port In1 of
the demodulator 1A.

[0223] The DP-QPSK signal 2 of the wavelength λi input from the
input port In1 is polarization-split into the X-polarization QPSK signal
(X-signal) and the Y-polarization QPSK signal (Y-signal) by the PBS 30.
Because the half-wavelength plate 40 is inserted into the optical
waveguide 9 through which the Y-signal polarization-split in the PBS 30
propagates, both the X-signal and the Y-signal are caused to enter each
of the 90-degree hybrid circuits 41, 42 in X polarization.

[0224] In the 90-degree hybrid circuit 41, the X-polarization LO light and
the X-signal are mixed and split into the I-, Q-channel components of the
X-signal. From the 90-degree hybrid circuit 41 to the B-PD 61, the signal
light of the I-channel component in the X-signal is output and from the
90-degree hybrid circuit 41 to the B-PD 62, the signal light of the
Q-channel component in the X-signal is output, respectively.

[0225] On the other hand, in the 90-degree hybrid circuit 42, the
X-polarization LO light and the Y-signal converted into X polarization
are mixed and split into the I-, Q-channel components of the Y-signal.
From the 90-degree hybrid circuit to the B-PD 63, the signal light of the
I-channel component in the Y-signal is output and from the 90-degree
hybrid circuit 42 to the B-PD 64, the signal light of the Q-channel
component in the Y-signal is output, respectively.

[0226] From the B-PD 61 to the DSP 65, a signal of a current value in
accordance with the intensity difference of the signal light of the
I-channel component in the X-signal (two kinds of signal light in
opposite phases) (balance-received I-channel demodulated signal) is
output. From the B-PD 62 to the DSP 65, a signal of a current value in
accordance with the intensity difference of the signal light of the
Q-channel component in the X-signal (two kinds of signal light in
opposite phases) (balance-received Q-channel demodulated signal) is
output. From the B-PD 63 to the DSP 65, a signal of a current value in
accordance with the intensity difference of the signal light of the
I-channel component in the Y-signal (two kinds of signal light in
opposite phases) (balance-received I-channel demodulated signal) is
output. Then, from the B-PD 64 to the DSP 65, a signal of a current value
in accordance with the intensity difference of the signal light of the
Q-channel component in the Y-signal (two kinds of signal light in
opposite phases) (balance-received Q-channel demodulated signal) is
output.

[0227] The DSP 65 reproduces a clock having the same rate as and in
synchronization with a demodulated signal output from the B-PDs 61 to 64,
respectively, by the clock extraction circuit and the sampling circuit
for the I channel and Q channel samples the demodulated signal with the
clock and generates a sampling signal. Each sampling signal is converted
into a digital signal by the A/D converter and a reception signal is
output from the DSP 65.

[0228] According to the optical transmission system having the above
configuration, the following technical advantages are obtained.

[0229] (1) Because the demodulator 1A in which the PBS 30 and the two
90-degree hybrid circuits 41, 42 are integrated within the PLC chip 3 is
used, alignment work and bonding work for the optical connection of the
PBS and the two 90-degree hybrid circuits are no longer necessary. As a
result of that, it is possible to manufacture the optical transmission
system 50 without connection loss between the PBS and the two 90-degree
hybrid circuits at a low cost.

[0230] (2) The demodulator 1A is used, which is designed so that the
effective optical path lengths of the paths from the PBS 30 to the output
port are the same in the paths of the signal light that enters the
90-degree hybrid circuits 41, 42, respectively. Because of this, it is
possible to realize a high-performance optical transmission system that
reduces the skew between the X-signal light and the Y-signal light in the
same X polarization.

[0231] For example, it is made possible to reduce the skew between the
signal light in the same X polarization to 5 ps or less in a DP-QPSK
modulation system optical transmission system with a symbol rate of 25
GSymbol/s and a bit rate of 100 Gbit/s.

[0232] (3) It is possible to realize an optical transmission system
particularly effective in optical fiber communication of the dense
wavelength division multiplexing (DWDM) transmission system.

[0233] (4) The current value of the signal (demodulated signal) output
from each of the B-PDs to 64 is in proportion to the product of the
amplitude of the DP-QPSK signal and the amplitude of the LO light.
Because of this, if the power of the LO light output from the LO light
source 56 is increased, the signal current from each of the B-PDs 61 to
64 increases in proportion to the square root of the power. Because of
this, it is possible to realize a high-performance optical transmission
system. Such advantages can be obtained by the optical transmission
system using the demodulator of the coherent optical transmission system
in which signal light and LO light are mixed.

[0234] The present invention can be modified and embodied as follows.

[0235] In the demodulator explained in each of the embodiments, a set of
the reception circuits including at least one PBS, two 90-degree hybrid
circuits, and an optical waveguide that connects them is formed on the
PLC 3, however, the present invention can also be applied to a PLC-type
DP-QPSK demodulator in which a plurality of sets of reception circuits is
formed on the PLC 3. In the PLC-type DP-QPSK demodulator configured as
described above, one optimum reception circuit can be selected from among
the plurality of sets of reception circuits, and therefore, yields are
improved and it is possible to make an attempt to further reduce the
cost.

[0236] In the demodulator 1D explained in each of the embodiments, which
is the demodulator explained in FIG. 9, it may also be possible to fold
twice at least one of the folded waveguides 43, 44, 45, 46, 47 to return
it to the original direction.

[0237] In the explanation described above, description is given on the
assumption that demodulation of the DP-QPSK modulated signal is performed
mainly among the polarization multiplexing coherent modulation systems,
however, the application of the demodulator of the present invention is
not limited to the DP-QPSK modulation system and the demodulator can also
be applied to other coherent modulation systems, such as QAM (Quadrature
Amplification Modulation) and OFDM (Orthogonal Frequency Division
Multiplexing).